You only receive the coins when the file contracts expire. Renters pay when they upload, but hosts only get paid if they store the file for the agreed amount of time.
The default period is 6,000 blocks I think, which is about 6 weeks. So, you should start seeing coins transferring to your wallet about 6 weeks after you first started hosting files.
Renters that upload through the API are free to set their own contract period (I have mine default to 2,000 blocks, which is about 2 weeks).
The important thing as a host is to remember to keep your host turned on. If renters cannot verify you still have the files at the end of the contract period (because your machine is turned off), you will not receive the coins! In that case, the coins are refunded to the renter. Oh, and keep your wallet unlocked too!