I've found in my career so far estimates are very useless but "key dates" can be very useful.
A key date could be an investor demo, marketing launch, actual physical launch if you are in hardware, etc, most importantly something with an audience outside of any of the core team. Find that important date and work backwards from it and find out what is important to finish by then.
This approach has defintiley saved me a lot of sanity compared to arbitrary "estimates" that are destined to be push forever until you run into one of these key dates.
A key date could be an investor demo, marketing launch, actual physical launch if you are in hardware, etc, most importantly something with an audience outside of any of the core team. Find that important date and work backwards from it and find out what is important to finish by then.
This approach has defintiley saved me a lot of sanity compared to arbitrary "estimates" that are destined to be push forever until you run into one of these key dates.