Seriously, get into knowing Amps.
Energy (what your battery stores) is related to voltage multiplied by Amps. Look at the Amps output of your mains charger.
Does it say 0.3A, 0.5A, 1A, 1.5A, 2A, 2.4A.....
Your battery will have a maximum input. Go look at it and see. It might 1A input or 1.5A input or 2A input.
You need to make the output of your mains charger match or exceed the input of your battery. e.g. if your battery has a max 2A input you need a max 2A output charge.
These are written in small print on everything.
For example the Powercore+ mini is a 1A input so most mains charges will do that, but say the Powercore 20100 is a 2A input. So allowing for power inefficiencies I'd expect 20100*3/2/2000 = 20100mah *3/2 ineffiiency / 2000ma = 15 hours to recharge.
Then add a little bit for the fact once you get above 85% charge it begins to charge slower.
You say it takes 2 days to recharge? I'd say look at your charger.