depends really, on the juice ratio you vape, juice types, ect, if you rinse them off ever, reburn them... Ive heard people 3-4 months on one wick and one coil even longer in some cases.
It's all really different...vg is thicker...but there is mesh for thicker juices too then you have juices that gunk up faster than others...it's all going to come down to trial and error really.