30 days plus a maximum of 3 30 day rearms for a total of 120 days before you have to use your product key to activate.
Assuming we are talking about a full retail key, you can use your product key for an unlimited number of future clean installs as long at the key is being used to activate only one installation of w7 at a time. Iows, you can't activate using that key on 3 different machines and have them all running at the same time.
Probably the above holds true for an upgrade key, but there might be a few more hoops involved, such as having a qualified OS installed on the machine you are installing to. (A previous activated W7 install would be a qualified OS.)
An OEM key is limited to one motherboard without a call to MS to explain why you should be allowed to install it on another MB.
After a certain number of reactivations within a limited time period, if any key fails to work, you might have to call Microsoft to explain the circumstances but that usually is not a problem.
