I went to Bressingham last year, and enjoyed virtually a full day there. Two of the railways were running, but the stock for the other one was visible. The standard gauge line doesn't look as if it runs very often, but the sheds were accessible.
As has been said above, there's much more than just the railways. There's a large museum, with road vehicles and tractors as well as quite a few engines. It's also the official "Dad's Army" Museum, and quite a few of the vehicles used in the series are there. I'm not into gardens and gardening, but it was quite pleasant to travel through them on the railways.
I'm not sure that I'd go again, because I've seen everything that I want to see, but if you haven't been there, it's worth doing so. One word of warning ... if you don't have your own transport, the bus service from and back to Diss (the nearest station) is not very frequent, but it is manageable with a bit of work.