Back in the day I would have said 10-10. In other words, 10% of viewers will watch and 10% of watchers will bid. However, that's for GOOD domains. No experience as of lately though.
In my opinion the amount of watchers doesn't mean much. I would take the amount of views more seriously. The reason for this is because I myself am watching hundreds of listing mostly for research purposes and I'm sure a lot of other domainers are doing the same thing.
Now that doesn't necessarily mean none of the watchers will bid. If I see a domain that I like and I see no one is bidding on, I might just bid on it in hopes of getting it at a good deal. And there's the big word "deal." If you're a flipper you want to buy the best domains for the lowest price possible to sell it at a premium and the majority of domainers on Flippa want to resell domains as fast as possible. There are those domain investors who don't mind spending a bit more to hold on to it to sell it in the future, but they're looking for really great names.
The times where you're listing will see the most bids is in the first 24 hours and the last 24 hours. If you list your domain with no reserve then there's a great chance your auction will get bids since it will be listed on the homepage under the no reserve section. You'll also get more bids the last 24 hours since your listing will end up in the ending soon section of Flippa.
So focus on listing really great names and promoting it as much as you can here on NP and other domain related forums.