Here is an older, but still nonetheless interesting article about Ven Thubten Gyatso, an Australian monk that went into three year retreat in his 60’s. He talks about the peace he experienced, and also for those interesting in doing retreat, he gives his advice on how to prepare for such a retreat. He mentions that favorable pre-requisites for such a retreat include:
Total devotion to a guru who will tell you when or if you are to do it, purification and accumulation of merit. Find a place like De-Tong Ling which is well managed for long retreats. Kimball Cuddihy, the center director, and his helpers were absolutely wonderful.