My preferred stage is “mid-to-late seed”, somewhere between $10k and $100k-$150k a month in revenue. If you’re bootstrapped or leanly funded, sometimes I can invest a bit later (e.g., I invested in Mixmax around $3m in ARR).
I don’t like to invest much earlier than $10k MRR because you don’t have product-market fit yet. That’s fine of course, I’m just not smart enough to see it when it isn’t there yet.
After about $150k in MRR, I might still want to invest, but often it’s a bit too late. My “check book” isn’t quite large enough.
More here: Investments (Prior & Current)