LMAO, 20 days.

R40-50 is 866k SP

Since you are stuck capped at grade 5 trees, you will be getting ~100 SP per success soon enough, and later on its 80-90.
Towards the end, your fatigue will hit between 15-20k SP for the day.

So, 866k, if you grind no less then 20k per day, which you can at first, but will not be able to towards the end, you will take no less then 44 days.

Just look at it this way, considering all SP to R50, R40 is roughly halfway.
This is made worse however for BTN/MIN because your options for gaining SP are lower, and only get worse towards 50.