Give three estimates. One if everything goes well, the most optimistically you can imagine. One representing the most likely path. And one representing the worst-case scenario. Let your manager digest from there.

If you have a bad manager (e.g. they would just use the best-case scenario estimate and ignore the others), average the estimates, 1 part best case, 1 part worst case and 4 parts normal case. Then try to make sure you are giving a set of estimates, not just one, and usually it will average out.

Step two of having a bad manager is finding a different situation, whether via internal transfer or a new job.