I personally think it can be individualized a bit, and can depend on other factors such as the spells itself, materials, etc.
Some spells specify exactly how long you should leave the candle(s) burning, and other details like that. If it does not specify, I think this is where it can be up to you a bit. Some people let the candle burn completely out. Others let it burn as long as they are doing the working, and then stop it afterwards. And some let it burn for several days, depending on the materials and the spell.
It doesnt really matter on most spells. Some people wait till the candle burns out and when the fire burns out the spell is released while some people just put out the fire themselves and it has the same effect. Unless the spell specifically tells you how long it needs to burn the time doesnt matter.
depends on the use. for the alter, those big thick candles you'd find and a pier one or some fancy home store. don't know why, but my favourite type of candles to use for the four elements when casting the circle are gel candles. not tea lights, but a bit bigger. they seem to last me 3 or 4 circles each... unless my friend casts who's a fire element, then the south candle burns out in 5 minutes flat lol. [not joking, it's annoying] for spells, it comes down to what the spell is and how much time i'm investing. for candle spells where it should burn out, i go for half of one of those candles you'd use in a candelabra. with a few exceptions, all mine come from the dollarstore, they have good sizes in my opinion.