I was you only a few years ago. I spent a year building a really complex SaaS in my bedroom. Then I tried to sell it the next year and a half with dismal results.

1) Validate first by talking with potential customers before building.

2) If the MVP takes longer than 3 month to build, you are barking up the wrong tree.

  a) problem/product fit is too high for a lone developer.

  b) your lack of experience & skill may be a bottleneck.

3) If people are going around saying it can be done in a couple of weeks, ignore them, unless they have done a similar project before in which I'd listen very carefully why it shouldn't take long.

4) If people are telling you've been at it for too long, it's time to reevaluate your self, skills & business.

5) Don't go at it alone. Partner up with a coder. Share the pain.