I've never put togeather an SS kit but I have tore my truck down a million times. Like 65Baja said 15 hours sounds about right! But take your time! The build is always extremely fun! Being meticulous in your build can save you lots of headaches later!
If you've got a good area and the correct tools...
About 20hrs to get something that drives.
The reality is you never stop making it drive better or adjusting for mishaps.
If you don't break it you're not going hard enough and then begins the slide to making it better. Same with all serious hobbies really, I'm still improving and playing with 35 year old pan cars.
The fact that you're on the forums indicates a desire to do it well, the journey begins, enjoy.
Being new, I presume you're contemplating purchasing one? The above 15-20hr answers seem accurate. But don't let that scare you away. If you don't have the tools or the time to build one, consider a new RTR or even something used. There are lots of advantages and disadvantages to each, so read up first on each before you buy.
I went RTR because it was an impulse purchase, and in hindsight I didn't have the time to build it just then
Some points to consider if you're looking to buy:
Pro-Instant gratification, just add gas
Con-You'll eventually need tools and time to makes repairs and upgrades
Pro-Lots of experience gained building it
Con-You need tools and time to make it RTR
Pro-Popular or unique grades are often done for you already
Cost, perceived cost and bang for your buck are important too, worth reading up on also before you buy.