It's impossible for us to work out a formula for it if we don't know how many exp points a monster gives.
You can make an estimate of how long it will take you to get to them maximum level though. Let's say that it takes you 3 hours to get from level 1 to 51. Assuming that you were killing monsters at the same speed, and you got the same amount of exp points per monster kill, it would take around 138 hours to reach level 90. It would take another 166 hours to go from level 90 to 100. At this same rate, going from level 100 to 115 would take 2,505 hours. This is a total of 2812 hours. If you leveled your chars every day for 4 hours at this rate, it would take 703 days, or 1.9 years.