My shrinking time in 6 miles (3+2+1)

Each week I time myself in 3 miles, 2 miles and 1 mile. I add those times together and track that total score. Here's how I have done over 14 weeks:

Starting Time: 120 minutes
Week 1: 115.25
2 - 111.5
3 - 107.26
4 - 104.3
5 - 101.49
6 - 99
7 - 97.28
8 - 94.08
9 - 92.08
10 - 89.27
11 - 87.32
12 - 86.23
13 - 85.29
14 - 83

So I have decreased that total time by 37 minutes in 14 weeks or 2.64 minutes per week. I want to see how long I can keep my decrease average above 2 minutes/week. I'd like to think maybe I can trim 50 minutes off in 25 weeks.