I don't change the water at all in mine and my plants are growing fine. I have nine 5 gallon buckets and it would be too much trouble and $$ to change every week. And if I really had to change it every week or two,I really don't see how it would be worth doing with that many buckets. My water never gets nasty though. I also stopped the recirculating system i had going for a month,changed to single buckets,and used the water from that system to put back into the single buckets. I do check ppm about once a week,and ph about once evert 2 weeks. My ph was around 7.5 for about the first 2 weeks and lowered it to 6.5 a couple times,and it just shot right back up to 7.5 the next day. So I just left it alone and my plants are doing great. The ph is now at around 6.5(after a month) naturally.
This is not giving you advice really,but just my example to help make a choice (because I don't have a lot of experience). So if my plants are growing green and the water smells/looks good then why fix what's working?