> For how long if I only try to mantain the time ?

Assuming a 32768 crystal, every Hz error in the crystal frequency
is going to result in about 3 seconds/day error. You should at least
use a trimmer cap for adjustment

Temperature compensation can be added to get an exact stable
32768Hz, but for long-term timekeeping I always use (and I've
mentioned this several times in the past) the driver chip from a
cheap kitchen clock. They are accurate, stable and low power. I
did see a manufacturer's spec sheet once that claims they are
trimmed to +/- 0.05Hz, which is borne out by the fact that every
timebase I've built with these is accurate to less than 1 min/year.
One time-stamped datalogger was actually within 3 seconds after
14 months and several others were under 10 seconds - which is
pretty impressive for an off-the-shelf (kind of) component

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