Cost, Time and Scope: Pick two. This is the typical balance that a product manager constantly has to deal with. The iron triangle of managing projects.
Time and cost are almost always fixed.
Most of the time, when you are building software, you have some date that is out there you need to hit. It may be a conference, sales commitment, training cycles, or corporate goals. Schedule is almost always fixed. There may be some flexibility on schedule at the beginning of the project, but often, once the first schedule is told to someone outside of the dev team, that pretty much sets it in stone.
With cost, most often this is the amount of dev resources you have. Most often, your dev resources are fixed. Larger companies can move resources around teams, but that heavily effects other roadmaps. Also, it takes too long to hire for a tactical project. Cost just doesn't change.
That leaves scope. Scope is the most important part of the iron triangle. This is the part of the equation a Product Manager thrives in. Setting priorities and knowing what needs to get released to best address the market is key.
Quickly figure out the schedule and the amount of dev throughput you have. Then spend your time on scope.