How often do you update your landing page?

When you modify your landing page? How you measure if the current one is behaving good or bad? I recently updated the landing page of my website https://watchnetspeed.com and within few days my sales are doubled. It's a pretty basic landing page but I need ideas for my next product's landing page. Most of the time, I do not change the landing page much, and it stays the same for a year. I think there must be some way to improve it.