local combi fare/Raising Wages

We currently pay our maid and gardener 400 pesos for their day's work, usually about 6 hours. They leave when their work is finished.

They have been with us for almost 5 years, and started at 250 per day, but within about 5 months, went up to 400. Our big discussion right now is do we go to 450 or 500?

Is there a guide for how often raises are given? He thinks it should be on a semi regular basis, but I'm leaning toward cost of living. We'd love some input.