If the hive has in March 10 mites,they achieve a critical level in September, 1000 mites.
Next month 2000 and douple every month.
It depends how long is you brood season.
Thymol and formic acid is ment to spring treatment.
If you have false swarm, it is good opportunity to clean the colony in Summer.
Hmmm. Michael's " do nothing" advices ... Brave...
Blanket statements are usually not correct in many situations.
This is why so many beginners are automatically told at beginner classes to treat hives every spring, every fall, use antibiotics without needing them, and all the other stuff. Usually from a crowd of folks that do nothing more than constantly treat by a "calendar" approach, while losing half their bees every winter, and then order packages every spring to cover their losses while complaining that they can't be "sustainable".
I've done thousands of sugar shake, mite boards, and other mite testing. And half the hives even in the worst yards do not need treatment. Contrary to the belief that all hives will have issues due to "leveling", and that all hives not one thing or another, will crash, that simply does not happen without extenuating circumstances.
You can have hives in your yard that are naturally dealing with mites and need no treatments. Other hives are less fortunate.
My advice.....there is no automatic or standard "advice" that all hives starting with 10 mites will have 1000 at some determined future date. Those that start to show mite pressure should be requeened. That means upgrading your genetics, as well as a brood break involved in the process. And if you can read the instructions on the package of chemicals, follow instructions, and do what is needed to actually complete what needs to be done by treating, you should be smart enough to find a queen and requeen a hive. Way too many beekeepers say "But I can never find the queen" and "I don't know how to requeen". But they can pour in chemicals easy enough. Maybe it's a "dumb and dumber" approach. :-D
I just had this conversation with a 1st year beekeeper yesterday. based on "advice" from some of the folks at a club, having him use terra patties, mite treatments, and all the other treatments....just in case! The best part was advice suggesting he use the inner cover between the brood chamber and the supers. Why? No clue. What happened, and I can only guess, but at some point after filling the supers last year the bees pulled back their area of defence to that below the inner cover. (They will shut the hole in these cases.) Then late summer all the comb was destroyed by wax moths and beetles. What a mess. And what lousy advice.
Anyhow.....Know what is going on in your hives. Have an IPM plan in place. Think it through. You should be helping the bees, while also allowing the bees to help themselves. Treat or act for only those hive needing it. And let the rest get stronger on their own.