I'm curious as to why you can't change your res? is it the expense of nutes or just the ordeal of moving the res and dumping it?

I'd have to say I'm on the fence between deftdrummer and zandor on this one...

I do see the point that if you monitor the res closely enough you should be able to go "longer" between res changes but that doesn't mean forever...for instance if you give just the right amount of nutes and have a really swank meter that can tell you concentrations of a bunch of different individual ppms you would be golden for a long time up to a month or 2 probably but I don't know anyone with the cash for a meter and setup like that (cause if you did you wouldn't be posting in this forum) so like zandor said the easiest thing is to change out before levels become toxic...I also have to side with zandor that the more res changes you perform atleast with all other things being equal the healthier and larger yield you will notice within reason...I can't see any benefit to changing more than once a week just like I don't see the reason to go longer than 2 weeks...with the exception of the first res change because young plants need less food and spit less toxins into the res so you should be able to get a longer acceptable res time the younger/smaller your plants are obviously making sure you don't notice smells or colors like deftdrummer mentioned

If it's the nute cost just buy a powdered nute to save money those work out to be cheaper and if it's physically moving the res to change it just by a pump that you can use to pump the stuff into another bucket...even if it takes a few fills of the bucket so it's comfy to carry it shouldn't take that long...besides than you could put a cleaning solution ( weak peroxide and water ) run through the res *look around for postings people have mentioned the strengths to use*...this will help treat the roots and clean the res and after a few minutes of running that you can just pump it out and then dump in your new nutes

good luck PM bigbud