It very much depends on your needs and desires. Many will say that after a couple of hours you have mostly explored the streets and the stores, and possibly a church or two, and then you are ready to eat something before heading back to Budapest.
The city has a great atmosphere, but it lacks additional programs, so unless you just want to relax, eat some good food, and drink coffee, without doing too much other stuff, you could stay for days. However, if you want interesting programs, lots of museums, a wide selection of restaurants, and other programs, it is better to stay in Budapest and only spend a couple of hours or half-a-day in Szentendre.