how many days it takes to learn after effect ?

462 days, non stop/  24 hours per day 7 days a week.

The simple answer is that you never can learn AE completely. Even people like us that have been doing this for more than twenty years forget about stuff or have never used certain features simply because they're not relevant for every workflow. As Warren already poijnted out, focus on learning what's axctualyl important to you and even then we're more talking about weeks and months to even come close to soem comprehensiveness rather than just a few days...

Mylenium

Completely?  While that's probably possbile, I'm not sure it's necessary.

I'd say a better approach is to focus on the features that best lend themselves to what you're looking to do with it from motion graphics to special effect or whatever it is in After Effects that might bring whatever is happening in your imagination to the Compsition panel and corresponding rendered movie or image sequence.