Their first question after you say this will be "well, how long will it take?" Whether or not you actually have any idea how much time you'll need, you'll have to give an answer, and on that blows the schedule up too much will be strongly frowned upon.

Now you've realized you're working for a crappy PM/EM who wants you to do their job for them, since they should have already known that the estimate from the first engineer will be nowhere close to the reality from the second.

The best managers I've had have been the ones that rarely need time estimates. They know enough about what's going on, and the comparative skill levels of the engineers, to give reasonable estimates themselves in most cases.