There is no 'safe' limit. Basically every minute a battery spends fully charged causes accelerated damage to the battery, it's only a tiny amount but it ads up. So it's best practice to limit how long a battery spends in the fully charged state. A few days once in a while wont do any measurable harm but if it's two or three days every week that very soon starts to add up: By the end of the year that battery will have spend in total 4 or 5 months in the fully charged state, which will do damage you can measure. The fact that the 4 or 5 months was split into lot of three day periods is not relevant, it's the total time that counts.
No point getting obsessive about battery care to the point it starts to limit your flying, but try to be as kind to your batteries as you practically can if you want them to last.
