I'd avoid worrying about the terminology here: whether this is a side-business or a startup. Your project is going to go through phases, and at some point you'll have to make decisions on time allocation.

Creating a minimal viable product can often be done in your spare time (time not spent at your primary job). It will take a significant amount of that time, including vacation time. Some product ideas may require too muc investment (time/money) to produce an MVP in your spare time, but possibly a prototype, or just a proof of concept.

If you're uncertain what your initial goal is, MVP, prototype, or otherwise, I'd suggest keeping it a side-project until then.

At some point you'll recognize you're not making enough progress, then you need to decide if you invest in it full-time -- as the first step to getting investment and potentially hiring others. If you are intending to make a living form the product you need to make this switch at some point.

It's a hard decision, and you need to have budgetted and planned accordingly prior to leaving your main job.

Not all projects require a full-time investment, it depends a lot on your goals. A lot of software is essentially hobbyist software, where people still work a primary job. Others have managed to get a flexible main job and dedicate more time to their side-project. Some people ween themselves off their main job slowly, until they have a revenue stream in their new venture.

If this sounds like I'm just waving my hands about, it's because I am. Every person and project is different. If you are serious about your product, then take the time to create a vision and a plan. What time and money you need should not be a mystery: you can figure this out with planning.

A few years ago I had a side application that did pretty well but I couldn't devote the time or money to take it to the next level. I think this would be easier to do when you're younger and don't (usually) have as serious family and financial obligations.

Another option is some kind of partnership if you can find someone that you can work with in this role. Some of the most successful developers I know have had a knack for finding the right business side people to partner with.

I quit my full-time job to start ServerlessOps with only a high level question in mind (What do I do as an Ops person if servers go away?) and a handful of business ideas. Those business ideas were categorized into short term ideas that I could use to generate revenue as I explored long term ideas.

I was able to do this because:

• I've saved a lot over the years and can bootstrap myself.
• I have a supportive partner so if I reach the end of my runway I still have a place to live.
• I have a supportive network, particularly around tech and startups

This isn't the case for most people but if it is then I say go for it.

Why? Because getting a company off the ground is a full-time job. There is a lot of work involved and lots of things you've never done before. (I've spent this week sourcing companies, packaging and pricing, finding out that I'm over thinking packaging and pricing, and crafting cold emails to leads.) When I was working full-time I had this idea I'd work nights and weekends, and if you are, then that's great but I couldn't do it. I was too burned out each day and while I had spurts of energy periodically, I couldn't sustain them and get real work done.

Additionally, let me also add there were legal reasons for me to decide leave my current employer. These mostly involved involved ownership of work. Even though I had gotten a second laptop, I wanted to make sure my previous employer couldn't lay claim to any of my work. You have an MVP.... I would sort the ownership of that out immediately and get it in writing from your current employer that you own it in full.

If you have the energy to work full-time and do a side project what I would have done as much work that doesn't involve producing IP as possible. I would have worked on defining the problem description, product offering, and initial business plan. Next, buy a domain (that you'll throw away) and put together a nice website that frames the problem, hints at a solution, and has a mailing list signup form. Post blog posts on the problem you're trying to define. Promote the heck out of that site and those blog posts. Then see if you get signups. You might even put up questionnaire forms for people to answer so you can validate ideas you have. This is all relatively "easy" work. If you get signups and questionnaire responses, then maybe you're on to something. Additionally, I'd be having coffee, lunch, or drinks with as many people as I can who would be helpful after I went full-time on my own thing. I was having those meetings on average once a week.

However, you built your MVP first and NOW you're trying to validate if you built the right thing. You can still use some of the tactics I described. At the least, clear the ownership of your MVP before you do another thing.