Hey Beck, I'm a bit of a noob here too. Not sure how well this applies to tomatoes and cucumbers, but...

1.) I've read that you don't want anything stronger than what would be ambient moon/star light. Anything stronger than that might cause the plants to think they should still be vegging. If you have to work on your plants during the dark cycle - use a green light as this won't affect the plant.

2.) In general I've read, most sources claim about 1 week tops - do able, but not recommended to go up to two weeks.. Copy and pasted below is a more scientific way of knowing when to change the res. It's the add back rule and the PH rule.

1. The Add Back Rule : Once you have replenished the original rez volume with fresh water, then it is time to change out the rez with fresh water and new nutes.

Never add nutes between nute changes,
until you have experience with the nutrients you are using,
and then you must have a tds meter.

Add fresh water once or twice a day to maintain the rez volume.

Do not adjust the pH before adding water to the rez.

Most of the time you will find that adding fresh water to top off the rez, will bring the pH back to normal range.

Check the pH and adjust if necessary, after adding water to top off the rez.

If you have a 5 gl rez, and you have added 5 gl over the last few days to maintain the rez level at the full mark, then it is time for a complete change out.

Keep and re-fill your plastic jugs to help maintain your add back count.

===================================================
2. The pH Rule :
There are separate pH rules for RO and tap water.

We will discuss RO first.

With RO, you will control the buffers.
There are three chemicals
that will allow you to get a stablized nute solution
without causing any nutrient lockout.

First you need to get some AN Barricade
which is Potassium Silicate.

You should also get some Potassium Carbonate,
and Potassium Hydroxide.

I buy these two chemicals premixed
in the powdered form from GH,
and then add RO to make a solution,
which I use to increase the pH with my eye dropper.

So here is the drill.

Add the nutes and any additives you are using,
and then measure the pH. (do not add Barricade until the pH falls below 5.6)

Most of the time the ph will be around 5.6
right after you add the nutes.

In a few hours check the pH
and it will be down as the buffer is overcome.
(I let it go down until 5.2-5.4
and then add Barricade to bring the pH up to 5.6).

Add one drop of barricade per gl,
and then check the pH.
( you need an eyedropper)

Keep using Barricade to bring the pH back to 5.6
until you have reached the max dose of .5ml/gl
(5 drops/gl of barricade max)

After you have maxed out the barricade,
then the next time the pH falls,
start using the GH Potassium Carbonate/Hydroxide powder in RO solution,
to bring the pH back up to 5.6
and after a couple of days the pH will stop moving.

You will have the solution buffered
with these three potassium molecules
and can reach a perfectly stable equilibrium.

When you add nutes to RO water, the nutes alone will probably lower the ph to around 5.6
If the ph drops below 5.6 after you have added new nutes to RO water,
use .1ml/gl Barricade to buffer the ph back up to 5.6.

Solutions with RO or distilled water
will require the use of an eye dropper to maintain pH.

Measure pH between each drop,
until you are familiar with your solutions requirements.

------------------------------------------------------------------
These are the pH rules for tapwater,
or water over 50ppm and/or initial pH of near 8.

After you add nutes to your water,
the pH should be close to 5.6.

If your water is very hard, over 200ppm,
then you will need to add pH down
(Phosphoric acid, or Nitric acid)
to achieve 5.6.

In a few hours,
as the buffers absorbs the acids,
the pH will move up,
back to near the original pH of the water.

So, you will need to continue to add pH down,
at least twice a day,
to re-establish the pH back at 5.6

After doing this for a few days,
the pH will become stable for a few hours,
maybe even for a day or so,
and then the pH will fall below 5.6 without adding any pH down.

When the pH falls without you adding pH down,
you have met the pH rule.

This is when it is time to change the nutes,
even if you have not met the Add Back Rule

For some nutes,
it is best if you always let the pH move from low to high,
and then you adjust it back down,
while other nutes will react in the opposite way,
depending on whether the mfg decided to buffer the formula or not.

Try to never allow the pH lower than 5.2 or higher than 6.2,
ideally you will maintain 5.6,
but in reality you will probably drift between 5.2 and 6.0

The time that this sequence takes will vary with the plant size,
the amount of water the plant is consuming/transpiring,
and the concentrtation of the nute solution (ppm/nute load).

If you are growing with tapwater,
the pH rule is the best method to determine when to change the nutes.
It can be as long as 10 days, and as short as 4 days.

If you must use tapwater,
It is highly recommended to use a Hard Water nutrient.

Your rez volume, the phase of your grow, and the type of nutes you have
will determine how often you need to adjust the pH.

Your job is to maintain the pH,
and change out the nutes at the appropriate time.
------

hope that helps....

that was great, thanx for the help/advice. I am missing 1 thing though. what is RO?

-beck

RO is Reverse Osmosis.
Most sites would recommend switching out every two weeks. Personally I go longer, adding nutes and water as I go. I've read where others say every week.
But I say what do you do with all that excess? If your system is small its not a big deal. If you're running a 50-100 gallon system, however, That's a lot of chemical to be disposing of. And it is not environmentally friendly to pour that much out in the yard.
I've also heard of the add back rule, though it didn't have that fancy name.