Rear ones last me 2 years and absolutely no tread left at all, the turf tyres with keratek.
The off road rear tyres last me 2 months, down to zero tread remaining, dont last 2 minutes...
Front ones? Down to canvas (then and burst tubes) around 2 to 3 times yearly.
I have no idea what you are doing to make them last SEVEN YEARS?
Front ones dont matter. The chair can get to the pub even when flat...
Rear ones mean you are stranded. So those are changed BEFORE the canvas begins to show...
Fronts.
Buy these in bulk. EBay, decent quality.
They last around 4 months a set.