I mean that would depend on your setup, 1 guppy in a 150 gal is not going to be producing a lot of waste, add plants to that mix and the Nitrates will take ages to build up.
But in all cases even prolonged exposure to low levels of nitrates can have serious implications for long term fish health.
Your water change schedule should focus on how fast nitrates build up in your tank,
If for (basic) example it takes 1 week to go from 10ppm to 20 ppm then a 50% weekly change would take you back to 10ppm.
But if it takes 2 weeks to go from 10ppm to 20ppm then a 25% change every week or 50% every two weeks would do the same.
Obviously it is more complicated than so there is a lot more to consider see some of @FishBR 's questions
But base your tanks schedule on how your tank performs not anyone else's
But remember nitrates are not the only reason for water changes. Some of my tanks have no nitrates ever. Won't bore you with the science but these tanks are too acidic for bio-filtration to work and I rely on plants exclusively to clear the ammonia, which means I never get nitrites or nitrates. I still do a large water change every week.
Like @seangee my tanks have many plants that take care of the ammonia so no nitrates or nitrites either but I still do 50% water change. I have close to 50 small tetras in my 55 gallon tank. I have my hardness at 40-45ppm but at the end of the week it is at 60-65ppm before I do my next water change. When my 30 gallon tank had only 13 small platy fry I did a large water change every 10 days until they grew bigger. All my tanks I now do 40-50% weekly water changes and 95% water change for my RES turtles. My fish and turtles are healthy-that is what is important to me.
I've heard so many times people say "oh they are fine" and meanwhile they have fish death all the time. Many people are self proclaimed experts. Even if a fish lives a year in these horrible conditions, this is only a short period of their lives being that on average most fish can live easily over 10 years under the right care.