Two hours probably won't be enough time for someone genuinely interested in Walt Disney. That's how much time I allocated for my first visit four years ago, and it wasn't enough. The building looks deceptively small from the front entrance, too.
If you really want to go through every exhibit, read most of the explanatory text next to the displays, and be able to pause and take in some of the more notable and rare objects, you'll need three hours.
If you want to do all that, plus see one of the movie presentations in the basement theater (shows vary, not sure what's going on currently) and stop for a snack at the café and spend 15 minutes perusing the gift shop, you'll need four hours.
Four hours is safe for a true Walt nerd, like many of us here are. And if you get done before four hours, there are plenty of other things to do near the Walt Disney Family Museum. Heck, just strolling the grounds of the Presidio and taking in the fabulous views is a wonderful time killer.
Walt Disney Family Museum - In The Shadow of the Golden Gate