How often should I be cleaning my filters? A Hot tub that is only used occasionally and is only 1 year and 3 months needing brand new filters? That concerns me. Hopefully the cleaned filters will do the trick.....I would be concerned the tub could freeze unexpectedly as the weather turns colder. There is no way of knowing for sure.
I’m not saying your filters need replacing and I didn’t know how old yours were. If the spa is only 15 months old then I'd expect the filters to be fine but you REALLY should rotate them after cleaning otherwise they aren't all "aged" equally. The one on the circ pump always gets dirtiest and if you never rotate then 15 months later that filter is much closer to needing replacement than the ones that never were on the circ pump.
As far as how often to clean the filters, I always tell people every month. Some are religious about it and don't have issues. Not everyone will adhere to a strict 4 week cleaning (nor do they necessarily need to depending on usage) so most probably clean them every 6 or 8 weeks and get by fine. Others hear the advice they are given and completely forget about them until they have problems with the spa and only after having issues do they become semi-good about routinely cleaning their filters.