does it vary for different coins? I'm asking because if I want to day trade I want to make sure its doable. Someone told me it takes a long time to have a buy go through and a sell.
if you mean something like placing an order under market price for buy and above it for sell, and waiting for that to be filled then there is no singular answer for it because it depends on a lot of factors.
first is the stability of what you are planning on trading. and that changes with time too. sometimes there is not much movement and depending on how deep your orders are, it can take forever for them to be filled.
second is the liquidity of that coin. some altcoins are too small, and if you place a big sell order in one of them, it will never be filled and you may even cause a drop with your "sell wall"
and finally the exchange you are using. some of them have less users and less liquidity specially in some specific markets for less known altcoins. obviously you will have issue getting your orders filled.