Well, when the plants are seedlings and even in early vegetative state you will likely not have to change the water/nutrients very often. It is recommended that you change the nutrients every 1-2 weeks. I try to push towards the 2 weeks but sometimes the plants will tell me otherwise and I may have to change nutrients at 10 days. It depends.

Once you get larger, flowering plants, or even late veg state, things change. At that point you will be dealing with water suckers. You will need to add water every week at least (in most cases). When you add water you will need to check your ppm/EC level and see what the nutrient strength is. If the plants took up a lot of nutrients with the water (they probably did) then you will need to top off your water with more nutrients.

I have personally never grown lettuce so I imagine those would be similar to seedlings/ early veg plants throughout the life cycle but I cannot say for sure. The above advice is for flowering plants.

The best advice I've read for beginners who aren't ready to invest much while they're getting their feet wet is this:
top of the tank with plain water only until you've topped off with as much as you started with. Then change the whole thing out.
Basically, if you have a 10 gallon rez, you top it with water only until you've added 10 gallons, then you change the whole thing.
It is not a perfect method, but basically works throughout the life cycle of the plant. as ethno pointed out, if the plants tell you they want more or less, take their advice. pale new leaves normally wants more nutrients. Tip burn (for lettuce) normally means less.
If you decide to stick with hydro, an EC meter and a pH meter will be short order purchases.

Thank you both very much for the help, and was exactly what I was looking for! I'm just starting with it, and will be more of a hobby to see how I enjoy it this year before trying to get anything more advanced.

Thanks again, and look forward to (hopefully) good experiences!

You can leave your pump on for 24hour a day; because of that you should not have to water from top ever again. When you have installed a new rooted clone; you need to keep the nutrient level touching the bottom of the basket. Whenever you see a root coming out of basket, make the level drop and maintain it one inch below the bottom of basket.

What is DWC or WC plz ?