An hour or two is usually sufficient and is extremely relaxing. The waters also ease stiff joints and rheumatic complaints. Most baths have a restroom, where you can take a short nap before changing back into street clothes. On the way out, tip the attendant Ft50-100. Apart from pools, saunas and steam rooms, most sites also offer massages.
Masseurs are professional but inattentive, chatting away to their colleagues as they work.
Tip them Ft100 or so. A full visit to one of the baths demands a whole morning or afternoon. There's usually somewhere in the foyer to get a cold drink, coffee or snack, plus a stall selling soap and other toiletries. Don't expect to have the energy to do very much afterwards except settle down for a long lunch or stretch out for a nap, but do remember to drink lots of water to rehydrate. Note that ticket offices shut up shop an hour before listed closing times.