it depends.
most people when they buy an altcoin, they keep it on exchanges to be ready to dump as fast as they like an also save on some transaction fees. it is worth mentioning that this is risky since exchanges can scam and run away.
as for transaction times, there are variety of altcoins with so many different attributes. some have faster blocks so your transaction will be confirmed that much faster. but also if you want to plan ahead you should check to see how many confirmation does the exchange need. the time can be from less than 1 minute to an hour depending on the altcoin.
additionally altcoins may have issues with their network. sometimes there is congestion like ETH and you can't even find your transaction. sometimes it is a 51% attack like on small altcoins and the exchange would extend the confirmation requirement.
here are the exchanges:
https://coinmarketcap.com/exchanges/volume/24-hour/