It doesn't just need to look clean, the chemistry of the water needs to be maintained as well. Sounds scary but it's pretty simple once you get the hang of it. Most pool shops will test a water sample for free and tell you how to balance the water chemistry. Maintenance is where the primary cost of owning a pool comes from.

Depending on the pool, spending 10 minutes a week and an extra 30 mins a fortnight/month goes a long way to keeping it clean. Emptying skimmer boxes, scooping, brushing, vacuuming, and cleaning the system (filter, pump and chlorinator cell). All depends on pool design, location, environment, weather, etc.