How long does it take to get customers?

We run a small consulting agency and have some great ideas for products but have no idea how long our runway should be.

So my question is, based on word of mouth alone, how long does it take for a simple web app to reach 100, 200, 300 users? (assuming our product is useful and has an individual/small company price point, $19-30/month)

I know it can happen in 2 days (Slack) or 2 years+. Anyone have any experience and/or advice?