It all depends on whether I play on-line or live games, On-line I can manage about 4 hours before my eyes begin to get sore, in a live game I could manage around 8 hours with regular breaks, it's not definitive but Old Age can be a bummer some times.
Playing Poker is always a drain on the grey matter and concentrating for long periods is taxing, but with regular 10 minute breaks I can get a breath of fresh air, go to the bathroom, or even buy a drink, it helps, On-line however is a bit more taxing because of the screen glare effecting my eyesight and the fact you only have 5 minutes to refresh your self, is a bit too short.
When the fun Stops, STOP!