Every 2 years
I had a couple of these and had to return one pair because of broken eye as well - got refunded BUT at that stage I had been experiencing huge discomfort due to total lack of waterproofness. It had become a joke among trekking partners to see how long I would stay dry. I suppose the shopkeeper was right when she said these boots were just not suitable for heavy trekking. I thought it made sense. I veered towards more costly but more reliable gear.