When is it too early to measure conversion rate?

This may come too late for you, but may benefit others who read this.

It's never too early to start measuring. It always helps to have data available and the measuring part is relatively cheap and easy. It would be nasty to find out later that you are missing data, for e.g. calculating your Customer Life-Time Value or some other relevant SaaS metric.

But whether to start optimizing? Unless you find some serious problems in your sales process, the best option at start is to focus on the things that brings you the most revenue.

You can use this tool I created to compare revenue using different traffic and conversion rate options: http://www.happybootstrapper.com/2013/traffic-conversion-rate-or-price-find-your-focus-with-projections/

It's not SaaS-specific, but you can get started with it.

It's never too early to measure. Measurement is pretty much a universal good as far as I'm concerned. Especially since starting early gives you the chance to get good at measurement before being good at measurement is really important (if you see what I mean).

What you need to be concerned about is how you make decisions based on the measurement.

It might be too early to (usefully) rely on statistical confidence for minor tweaking - it depends on the significance of the results that you see. VWO have a handy online tool for seeing whether things are significant or not statistically http://visualwebsiteoptimizer.com/ab-split-significance-calculator/ - experiment with the numbers to look at the kind of thing that might be useful for you.

What might get you some more bang for your buck is doing some quick and cheap user testing with, y'know, actual people ;-) If this is not something you've done before I'd thoroughly recommend getting a copy of Steve Krug's Rocket Surgery Made Easy.

Also - remember that you don't always have to rely on statistical significance. If you see two people walk past and trip on a loose corner of carpet - you don't have to wait for N other people before knowing that it's a problem and you should fix it.

It's never too early to start split-testing, but lower traffic could mean it will take longer to see results. It all depends on how large the difference is between the two versions.

Use a Chi-Square calculator (like this one from usereffect) to compare the results from the A/B testing of your landing page(s) and see which ones convert better. It might take 20 conversions and it might take 2,000 but you won't know until you try.