How to Calculate Payoffs?

[Luke Carl]

My brain isn't wrapping itself around this one so I thought I'd ask for help. 

All hypothetical. 

Lets say I have 1 property with a purchase price of 125k and I have a mortgage on 100k at 5%

I'm netting $1000 per month. 

I apply all profits to principle. 

How long does it take to pay this off? 

The bigger question is... If I scale to lets say to 5 properties all with the same numbers.... 

How do I calculate how long it takes to pay off the first one (weather its in years/months or some sort of ratio) using the net profit from all 5 of them.... Then to pay off the second one with the now larger net profit from the first one and from the other 4?