This is by far the most time consuming achievement in the game and will likely be the only achievement standing in the way of you unlocking the achievement for having every achievement.
At the completion of every other achievement in the game I had around 40,000 Ranger Points.
You need 81,000 Ranger Points to get the final badge, Heart. (The achievement popped for me at 81,283 after gaining 500 or so points in training, so I'd say it's 81,000).
You can track your badge progress from the Collection option on the main menu, then select Samurai ID, then Badges. It's the only badge in Case 6 on the collection screen, so consider it your goal to make it that far.
For me, it was a quite a grind to get there and I found the easiest way to get a decent amount of points was to simply repeat Ranger Training Level 2 over and over again. Each time I obtained anywhere from 460-560 Ranger Points depending on my performance.
If any one finds a better, faster method to grind points let me know and I'll update this solution.