On a Chase UAL biz card which was put out to pasture after getting the minimum spend for bonus miles, a few months wait garnered a triple miles offer. Card was extensively used during triple miles offer which lasted two months then put away again. Now, a new double miles on six categories offer, gas, groceries, etc.

Other UAL cards which were used a little got no special offers.

Conclusion: don't use card for a while, get more miles.