The Runequest sheet took me about 3 weeks and the COC 7th Ed sheet before that took about 2 weeks. I stayed up late a few nights but I wasn't working on them every day either.
That said, I haven't really started to learn html/css until about 3 months ago when a change in job role required me to pick it up; however, I did have a background in Computer Science and experience with various programming languages. Most of the work on character sheets is not overly complicated but there are tons of elements, so there is a lot of copy and paste, tweaking auto-calc equations, etc. There is also a lot of positioning work as you try to fit all those elements into the page. Its actually a really good way to learn CSS/HTML, especially the layout concepts.
As for help, the existing sheets were a good starting point(https://github.com/Roll20/roll20-character-sheets) and Roll20 has a forum dedicated to sheets(https://app.roll20.net/forum/category/277980). It's probably worth checking the forums to see if anyone is already working on sheets for the systems you want and/or is willing to help make one.
Also I had the hardest time getting started because I didn't read the finer points in, https://wiki.roll20.net/Character_Sheets. Special attention to the parts about transparently prefixing elements with 'sheet-' if your CSS is not working.
Hope that helps.