Strange stones are automatically converted based on max value for the current day/week. As soon as you go over, then it triggers the conversion.

Sun converts at 25:1, and moon converts at 100:1. Sun coins are always 5800 max per week. (This may change in the future, but right now this is the weekly max for every player in the game.) So whenever you go over that during the week (doesn't matter what day it happens), then the rest converts to strange stones. Same goes for moon coins, but that depends on your level and if you maxed out your previous weekly cap. Moon coin limit increases every day by usually 4K coins or more. For higher level players generally it starts around the ~40K range, and will max out around 64-68K if you always max moon coins for the week. If you miss maxing out the week, half of the amount you missed in the previous weeks gets added to the next week's cap. So if you missed 10K moon coins from your max cap on sunday, then on monday, it'll add 5000 to your max cap for the upcoming week.

There is no limit to coins in your bank. You can save 1,000,000+ moon or sun coins if you want. That just requires weeks/months of not spending them though. Which isn't recommended.

That's literally what I said....

Ahh ok. Carry on.