5m would need about 4-5 days of coin age so that part makes sense yet. Staking for max rewards is most definitely wasting coin age though
I was under the impression the most you can get in a stake is 10% of the block #. Block number, let's say 1.1m, would be 110k.
Highest APR is 500%, or 1.37%/day.
8,000,000*1.37%=109,600, which is close enough to the 110k. So that's 8m coins. At 5m in a single stake, it would be 8/5=1.6 days for the maximum stake (assuming 500%). Assuming you get half that, 250%, it would still just be 3.2 days for the cap. The BEST method is to stop it at 1.6, just because it encapsulates all other stake % possibilities as well and ensures no matter what's chosen, you aren't losing anything.