I wouldn;'t use JUST a charger, as they can over charge things if they don't have a shut down when fully charged
as for HOW Long between use and being on a trickle charger, this will honestly come down to WHAT ATV you have and what all is on it and was used
older atv's are LESS Likely to DRAIN a battery sitting than new one's
my 1999 400 foreman, sat for its first 14 yrs or so in a 3 sided barn with NO power, , temps in barn dropped well below freezing in winters and above 90's in summer, would run atv some times a few days a week and sum times would sit for a few weeks, winch got used often yr round,
NEVER had an issue with a LOW battery,
and averaged about 5-6 yrs out of a battery, NOT on a charger and NOT ever charged after any use, just ran and parked it, and it always started , unless battery was DEAD dead! LOL
ATV's at my hunting camp, of mixed brands and sizes, will EAT a battery in a few weeks left parked< too much gadgets on them that suck power when parked!
its ALWAYS best to have hooked up to a trickle charger if you can, will add a yr or MORE of life to a battery, than one that doesn't have it on!
but its NOT a huge difference again on older atv's that just don't have a ton of electric robbing items on them IMO!