The answer is it all depends.
If you are targeting SMB's with your service then you will need to approach a lot of them. If you are using ads then you can expect likely around a 2-4% conversion on average for a decently targeted campaign, 10% if you are highly targeted. Remarketing/retargeting can help bring those numbers up too.
As for where I get my numbers, we run our own SaaS platform as well as we have developed a number of platforms for other people and in some cases are still maintaining.
You can read about the slow ramp for SaaS businesses a number of places, I believe patio11 has also published some information on it even and he's always a good resource.
In the end you have to test and market to see what works best for you. In the beginning expect to be much more high touch and to do personal sales, as you can ramp up and get more word of mouth it helps snowball new signups and reduces the amount of time you spend on the phone personally signing people up.
For my 2 cents, talk to as many people as you can everyday and try to sign them up on the spot where possible (if even for a free trial). If you aren't doing free trials then try to find the key thing that helps sell your product and do demonstrations on it, and make a video of it (less than 90 seconds) and send it to everyone that won't sit with you for an online demo. You get the idea. Also, if you are still the one writing code and trying to do sales, pick 2-3 days a week where you focus on sales, the other on development/support. Or split your day where you write code/fix things part of the day and spend at least 2-4 hours on sales efforts initially per day.
As for myself, when I talk to people one on one, I can generally get a very high conversion rate > 50%, but when you rely on marketing only that drops considerably. However, the more targeted your audience the better your results will be.
