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.
Why are you asking this? The process is instantly, just take a look over the exchanges, and make a trade, you will see that it is instant because on the major exchanges there is a huge volume everyday.
Now if you go to a weird exchange, your trade will be processed slowly, because probably no one is going to buy/sell your alts in there.
That is why a lot of people are just using bittrex/cryptopia/poloniex, because they have a lot of volume compared to the other exchanges.