When to change filter in water purifier?

Outdoor filter every 3 months. inside each year

You will find information mentioned on the filters itself. In general all RO, UF, Carbon sediment filters need to be changed after a year. Pre-filters need to be changed every 3-4 months depending upon the water source.

It depends on the quality of the water. Purchase a TDS meter and test the water quality every few weeks also check the manufaturer's user manual.

Better to buy AMC which includes periodic maintenance visits and filter change once yearly after 1 year

Repeating @Gulag-Survivor's words... It totally depends on TDS of input water. Buy a good tds meter and keep checking the purified water on monthly basis.

If your input source is corporation water, then TDS of it would already be around 150. In that case you won't need to change even the pre-filters that frequently. But if input water is from bore or pvt water-tankers, which have high TDS, then pre-filters would get exhausted quickly, hence need to be checked every 3 months.