Yes, been there loads of times - great place to kill time midweek.
Re how long you'll spend - depends how old your kids are.
Basically, it is a two part garden centre (drive right around the back if you want to be near Butterfly world).
At the back in a very big glorified shed there is a butterfly world which will take you 30 mins to go round and a small animal area. Next to this is a small but good indoor play barn which is great for toddlers but less so for older kids (by the time DS was 4 he found it boring). Around the edge of the play area are a number of indoor craft units, including paint your own pottery etc. Then you can walk through to the garden centre which is typical of Wyevales, some toys, plants, fish etc.
There are two cafes, one near butterfly world and a smaller (but nicer) one in the garden centre itself.
My advice GET THERE EARLY. It is very popular at weekends.
If you included lunch and your children were the right age for the play area, I'd say you'll spend 2-3 hours there.