I am also not trying to be harsh but,
it's not ok and it's not going to be ok.
if you have to put tires on once or twice a year something is drastically wrong. lets get some more info,
what make, model van is this?
how much does it weigh? front and rear?
what brand, type, and load range tire are you running?
how many miles per year do you drive?
I want to emphasize again something is seriously amiss here. under no circumstances should you be replacing tires that fast. in a situation like this you are not only endangering your life but others on the road with you.
highdesertranger