We ran our side-project, https://emailoctopus.com, for around 3 years before going full time. As a low-cost platform, our sales are quite low touch, so we'd be able to set up our marketing projects (Facebook ads, email campaigns) in the evenings and just let them run on a schedule.

Support was a little bit more tricky, however. The only solution we found here, other than outsourcing to an Upworker, was to try to minimise support. Make your help docs as useful as possible and spend time on improving error messages.

I would also advise you to switch to working on the project full-time as soon as you can afford to. Our growth went through the roof (we'd spent around 3 years getting to £1k MRR and tripled that in the first full-time month). Some reasons why? We could spend time with our customers and focus on improving our metrics, the stuff that you just can't automate away. We also began treating it more as a business and valued our own time spent on the project more, which resulted in increasing our prices and getting across our value proposition better.