So how long do I need to keep the wallet locked with 2.2M blocks?
Depends on stake rate. If you get a 500% rate, you need a lot less time than if you were going to hit a 1%
It's impossible to predict what you will receive until time of stake. Therefore, it might be best to just use one giant input if that amount can make you stake higher than the maximum reward per block
so change the rules again

, so to get guaranteed reward(although probably more important not to receive penalty),
need 10 million cube >360M coin age(in one input)!!! with such a policy you will lose the last investors...
p.s
I totally forgot here after all the possible 1% of the annual interest and penalty 10% daily

what will risk ?! possible reward 0.00274%-1.369% again possible lost 10%...