I've been following the Stronglifts 5x5 - plus HIIT right afterwards - for about 3 months now, gone paleo for 1. I just hit my first plateau on bench, 65kg. Now, though I am training at what I'm guessing is my bodyweight (no scale), could the plateau be blamed on muscle glycogen depletion, as well?
Glycogen is not the first thing I think of when I hear that you've hit a plateau.
It may be that:
You are not eating enough You are not eating enough protein You are not sleeping enough Your form needs to improve You are doing too much HIIT You need a back-off week to recover more completely
Or any number of things. It's difficult to give you accurate advice unless you provide more information about previous training and athletic background, your height and weight, diet composition (calories and some macronutrient estimates), sleep schedule, stress levels, and maybe post a video of your lifts for critique. =)
The HIIT could be to blame, as it causes a signal opposite to that of muscle building to be sent out. Any HIIT/LISS should preferably be moved as far away from resistance training as possbible.
After that, making sure you get 1) enough rest, 2) enough protein are the two important things.
Stronglifts 5x5 has no periodization, which could be useful in your case. Have a look at Korte 3x3 at http://www.fituncensored.com/forums/sistemi-di-allenamento/128-korte-3-x-3-revolutionary-approach-powerlifting.html (text in English)
Nick, RE: muscle glycogen, it usually takes about an hour of steady activity to deplete muscle glycogen. Glycogen is usually not a factor in resistance training, unless you train after a hike, bike ride, etc., or train for more than an hour. How many days a week are you doing resistance training? Recovery is an individual phenomenon, but some people need as many as 3 to 5 days between resistance sessions for proper recovery. It could take about 3 months (the period you've noted) to hit an overtraining plateau. Also, I agree with Mikael that HIIT should be separated from resistance work. It doesn't sound like a glycogen issue.
Congrads on the 1x bodyweight bench press.
There are a number of things that can make your bench or other lifts plateau, glycogen depletion is an unlikely culprit.
A more likely culprit, as Mikael pointed out, is program staleness. Try out some other rep schemes (8x3, 6x4) and alternating your rest periods. Remember volume and intensity are inversely related.
It is also likely you have weak external rotator strength. Poliquins data suggests you should be able to perform 8 external rotations with around 15 pounds.