This depends on a number of factors, including the fiat onramp and the payment method used, as well as your location.
Generally, card payments are processed almost immediately. Most crypto arrives anywhere between 10 minutes and 60 minutes from the moment of submitted the transaction.
It’s not uncommon for some transactions to take a few hours. Some bank/wire transfers and usage of ACH can take anywhere between a few hours, up to 3 (business) days.
In cases of heavy network congestion (which can happen with Ethereum), the on-chain transaction might take longer to execute. Bitcoin generally takes up to an hour to arrive.