50 feet or less is a good rule of thumb. Otherwise you start playing with the limits of the device and dropping the local connection to your devices.
MiFi's are condensed and battery powered routers. There is not alot of space for them to hold the equiptment necessary to provide a stronger signal. If you are looking for a higher powered signal then check into products called network extenders or repeaters.
If you need to go over 50 feet like John pointed out, you will need an extender I am partial to the PepWave Surf Mini 200 mW radio or the PepWave Surf on the Go.
The PepWave Surf mini is like $69 bucks I believe at 3gstore get the external antenna and you got 3/4 of mile range. IF you really want to blast a full 2 Watts of Power get the AP One. you can view more info at 3gstore