Answer to your second question - I use piece of mosquito screen; have used drywall mesh; you could buy a plastic mesh in craft stores (I think they call it plastic canvass...) or even ordinary coffee filters.

Last time I had my screen in my patio door replaced, I kept the old screen. That's what I use to put over the holes in the bottom of containers. I've also used old worn out landscape fabric. I have enough screen for quite some time thanks to my door. If you're ever in need, usually people just toss it out in the trash and you can just pick it right up!

I saw the coffee filter idea the other day and thought that was pretty cool too. But

A couple of years ago, water stopped draining from two fairly-large (16-18") containers. I washed the old potting mix from the roots and repotted the plants in new potting mix with the drainage holes covered with a piece of window screen. The plants having be doing great ever since.

I have many other smaller pots (8-12") containers that I should do the same with.

How often you repot is your choice, but here is a rule I go by. You can get away with potting up indefinitely, as long as you pot up before root congestion reaches the point where the root/soil mass can be lifted from the pot intact. Once that occurs, the plant will be permanently limited until the root issue is corrected during a repotting/root pruning session - even if the plant is planted out. The limiting effects of root congestion increase in direct relation to the severity of congestion.

Al