Holocaust Memorial Center and the Great Synagogue

It's fairly extensive , three hours would work

The standard tour of the Great Synagogue (Dohány Street Synagogue) is maybe 30 minutes. But then you can hang around on your own. So figure an hour at a minimum. The Holocaust Center is pretty large. Hard to imagine getting through in under an hour. But then you have to get from one to the other and that's maybe 30 minutes on foot or 20 by foot plus the M3 metro line. But dont dispare, the route if you go by foot is very interesting and you can stop at the National Museum along the way. Or you can ride the 47/49 Tram one stop and save maybe 4 minutes and rest your feet a bit before hoofing it again.

So for the two, Steven's 3 hours is about right.

But there are a number of other synagogues worth visiting. https://www.offbeatbudapest.com/budapest-city-guide/jewish-budapest/synagogues-cemeteries/