In my experience there is more than one or two factors actually affecting how long your brush lasts. From the kind and scale of the minis you paint to technics you use to consistency and brand of paints to actual quality control at the factory and last but not least care.
It is said that metallics degrade paintbrushes faster, same is true for inks and washes (and I would expect contrast paints) , but I guess it comes down to how much of those types of paints you use. I don’t use metallics almost at all and still get maybe 80-100 28 mm minis out of series 7 (using soaps and all), from what I understand you paint more than that in a year.
Back on topic thou, I recently started painting with oils and use cheap synthetic brushes with them, granted technic is much different, but after 2 or three months (15+ minis) I haven’t noticed any degradation on those no-names:
In Dublin the set costs less than 4 EURO ( https://store.evansartsupplies.ie/product/20461/Brush-set-Short-Handle-10s/ ) . I used them with acrylics as well, and they aren’t brilliant, but sufficient for some base colour work.
