Over two years with over 400 hours. R4's all around.
No flats on the rear tires. On the front, got the first flat about two weeks after I got the tractor. Used the "whatchamacallit" and gave it a shot of sealer. About a week later the other front tire went flat. Gave it the same treatment.
Since then I ocassionally have to air up the tires, and every few months give them another treatment. Only once did one pick up a screw, had to plug that hole. All the rest are from thorns, thorns, and more thorns. I have no idea, how many thorns are in those front tires, or holes from thorns, must be in the hundreds, at least.
Ernie