daily or at least every other day...
i have a mating pair in a 28 G and they are happy as can be, 2 plants, bare bottom, and a rock. changing water every other day will be a minimum though, they poop alot and it makes a mess, especially during feeding. i would recommend getting a larger tank than a 35, because these fish are addicting, you WILL want more, trust me. but be prepared to make a commitment to thhose fish.