Yes, we are not talking about many tens of thousands of messages. The mailboxes I'm interested in to sync on Android are maybe 6-7000 messages combined in each (account).
I guess I'll be better off to set the limit by number of messages. I'd just wish to have age-defined limit for more than 120 days (150 or 180 as a compromise), but that (by number of messages) will do as well.
By the way once you sync all your messages (or the last 6 months) in the app, they are cached and even if you skip several days, you will have to sync/download just the new mail after you connect and sync.
Especially for mailboxes that are not too busy. ADDED: Say, if we have 10 000 messages, set up 16KB retrieved per message, this is avg. 100MB, which is nothing to todays phones. With the other client I managed to sync all my messages locally this way, and the phone (2GB RAM) didn't even flinch. Even the scrolling was smooth all the way.
Anyway, thanks for the best email app for android, and I tried all of the known apps. Been using K9 as a good compromise for year and a half, but buying Aqua Mail was an easy and fast decision.