ROAS is worst (and ACoS typically higher) when campaigns first launch.
To get conclusive data, we don't remove keywords until they get 10 clicks and 0 sales.
Also, we test a minimum of 50 keywords per ad group and optimize them over time.
With this in mind, on average it takes ~3 months with spend of at least $100/day to begin seeing good ROAS. The time period varies depending on the strength of brand, listing optimization, history of campaigns, etc.
We understand you don't want to lose money in the beginning. That said, it's hard to run PPC correctly without investing in showing up on the first pages and getting data.