Impossible to give you a perfect or precise answer.
1x 50% weekly Is typically recommended. Or slightly larger. In a 10 gallon tank, that is 1 bucket worth per week, not too much elbow grease required for that.
Not only for keeping NO3 in check, a 50% or larger water change will keep water parameters stable, especially if you top-of evaporation with tapwater, because the TDS will exponentially increase as you continue to add mineral rich tap water into the tank. If you change over half of the tank water, you will be above the " law of average" and thus lower the TDS number to more closely reflect your tap water readings, thus maintain a consistent set of parameters.
If your tank is cycled 50% once a week will be good for guppies. It depends on what fish you have too. Some fish poop more than others. Guppies are light bioload, goldfish high bioload. etc. Get male guppies so they dont explode in population on you.
It's recommended that you do a WC every week. Beyond that, there are too many variables to give a precise or exact answer.
Here's what I do. I always do a 75% WC. I test my water 7 days after the last WC. If my nitrates are less than 10 ppm and ammonia and nitrites are zero, I will go at most another 2 or 3 days before I do the next WC. If my nitrates are higher than 10 ppm, I do the WC on the 7th day. Since my tank was cycled, I've never had ammonia or nitrites above zero, so that's never been an issue, but if they were wonky, I would do an immediate WC to get them back down to zero, or as close as I can.
I just added a piece of driftwood last Saturday, so I'm seeing a lot of tannins. Though they are harmless, I'll be doing a WC every 5 to 7 days until the wood stops leaching the tannins into the water.
I agree with all the great information above. I usually do a water change once every 5-6 days or when ammonia readings are high. Guppies have a pretty low bioload, so as long you are not overfeeding you should be fine.
Unless you have another tank to keep the fry in, I would suggest all males (for their colors). Guppies breed quickly and most fry survive, especially with plants, so just a heads up on that