If you're using a hot tub or spa, water should be checked three times a week. The latter is especially true if you use chlorine, or a mineral purifier. In peak periods such as summer, water levels in a swimming pool should be checked on a daily basis. Knowing how to clean the filter in advance is essential.
According to Art of Poolcare (artofpoolcare.com.au) the frequency is dependent on the pool size, pool location (e.g. if it is surrounded by trees with falling leaves), the pool usage and the season of the year. Pools generally need to be serviced more regularly in summer than winter.
When we did have a pool, I would check the ph and chlorine at least once a day. Of course, I would then treat the pool if needed. When the problem became out of hand, I would call in a pool guy to sort it out. That generally was about 3 months, but varied according to needs.