I think that with the countless innovation in this business, its a crime not to provide either some kind of a free trial or a money-back guarantee, which is at least 15 days.
With startup companies popping like mushrooms after rain, its important for the customer to get a feel of the service before they decide if it will satisfy their needs. This is also a chance for the company itself, to prove their value and uniqueness and gain a useful customer base. If you are sure in the quality of your product, then why not share this confidence with the rest?
Although even a couple of days are technically enough for a person to check most important qualities and features , it does not give you the sense of space and freedom to really get a feel of the host. The customer feels pressured to try everything at once and loses the purpose to keep an eye on details. Even a week seems to me as a short timespan - user can be busy or have urgent matters to attend to and the 7 days will pass in vain.
Bottom line is, should a host wants to build a reputation based on a quality product, they should give enough time to play around with it. If you start with the attitude that you don't want people to "freeload" your service for a longer time, then you probably won't get many people sticking around too :rolleyes2