It's pretty long.. stops at major cities along the way such as Rotterdam, Den Haag, Antwerpen...
It's not too bad of a ride, however. It's fairly scenic (as far as crusing through the costal flats of the Netherlands can be), they sell drinks and snacks which are pretty much market priced... I did it and didn't mind.. slept part of the way as well.
The thing about Thalys is that it stops at the Brussels south station... 3 min subway ride in BRU but still..