9 months: January to September. It took 2 weeks after launch to get the first paying customer.
The product is a SaaS progressive web app for budgeting. It's $12/year for the premium version.
The main challenges were adding the last few features, making it production ready, and polish. The 80-20 rule really applied here (it took 20% of time to get 80% of the way), and that's only for the MVP! I still work on the app almost every day. :)
I'd say the hardest part now that I have many customers is not breaking anything for those existing customers.