"Can you advise me? My baby has been latching on for about a week now,…

"Can you advise me? My baby has been latching on for about a week now, and he's been sucking for 3-4 hours at a time, crying if I take him off. What should I do? Has anyone experienced this?" "It happens that in one feeding everything is fine for 20 minutes, but in another it lasts for hours and he cries even though he's still sucking."