Every time you retire you gain experience 3% faster. So on x1 your first generation, you have 1000 experience a tick, x2 is 2000, x3 3000, x4 4000, x5 5000. At gen 2 it goes 1030, 2060, 3090, 4120, 5150.
Im into my 11th gen and my x5 XP is 6650 every tick. This keeps doing this until you hit get gen 16 then your XP bonus stops going up.
When I was early on in my generations I was able to retire within about 3 weeks of playing about 12 hours a week (maybe closer to 15). It also greatly helps if you are able to get into some large strategus battles. Some of the larger strategus battles will give 750,000 - 1,000,000+ XP for an hour and a half of fighting. On average, the 1500 man sieges in strategus usually give at least 400,000, and up to 750,000'ish XP. You can retire very quickly like this.