You can totally do another "cycle" of SL! As a matter of fact, it's best to try to max out your ability to consistently add weight to the bar before moving on to another program; often times you would be moving to an intermediate program.

I'm just finishing my 24th week of SL, rather cycle 2 as I call it (I'm strange like that lol) I did make some modifications in cycle 2 though. I went to 3x5 and also stopped adding weight every workout. I really concentrated on form and usually increased every 2-3 workouts instead. I also played with increasing reps instead of weight....so basically in each workout I was still progressing by some standard. I also found that switching to 3x5 gave me a little more energy to do some other things I love. I now add a finisher onto the end of each lifting session and I do cardio on my non-lifting days. I have been super happy with it!

Every program has some sort of a time frame attached to it....12 weeks is a pretty standard period of time to run a program. With that being said, it's also not the "max" amount of time that you can run the same program. If it's working and you are seeing gains, then carry on.

Yeah, I've been doing it for quite a bit longer than 12 weeks and don't feel ready to move on to an intermediate program. I also have modified things like Vegas (I do 3x5, and sometimes I don't increase weights every single workout - depends a little on the lift and if I feel totally confident on the form, that kind of thing.)