Overhaul rewinds the maintenance clock at 3x the unwinding rate, meaning 4 months in overhaul reduces the maintenance clock by 1 year. So a ship needs to spend 1/4th its time in overhaul (if it is away the rest of the time). As for the cost, I don't know but it's quite low.
That's what it was - the clock is unwinding at 4x, while time marches on at 1x, for an overall unwind rate of 3x. (It makes sense - really it does
).
Just noticed OP asked if there's a downside to overhauling more frequently. The only one that I can think of would be that once the parts are scattered across the deck, it takes a fixed amount of time to get out of the overhaul state IIRC. OTOH, if you overhaul frequently enough you'll usually be laid up for less than the "abort" time. And since the breakage rate increases as the time-on-clock goes up, there's a strong advantage to overhauling every time you come back from a mission (as Garfunkel alluded to). I just am in the habit of giving refuel/overhaul orders when returning to port - I think most people do the same.
The exception to the above is if you are in the middle of a war and have warships for which you can't afford the downtime, of course. Eventually it will catch up to you, though.
John