If the artificial grass is in a relatively flat area with minimum traffic, your infill should last you years, if not a lifetime. Here are some factors to consider that might indicate the need to add more infill:
1. The artificial turf is on a slope that will allow rainwater to wash away infill.
2. If the turfed area has a lot of recurring debris (sticks, leaves, etc) that requires regular blowing or sweeping, some infill may get lost over time.
3. Heavy traffic can matte down the turf and carry away infill.
If any or all of these 3 apply to your space, don't sweat it. Your local FusionTurf dealer is there to help. Your local dealer will be able to address not only if you need an infill replenishment, but also the correct amount to add.