It all depends...
If it is hot out and the tank is out in the sun it could lose half its strength in a week. Unless you are one batching your surfactant than the degradation will be expedited.
Using your injector will keep the bleach contaminant free. We had a full drum of bleach that was from last thanksgiving and this first week of April it was still practically full strength because of the cold temps keeping it fresh.

Once it gets warm try to park in the shade when not using your truck. Like over the weekend for example.

I agree with Zach, I also had bleach left over from last year, had no issues with it when I used it this spring.

Thanks for the input. Mostly I am concerned with during the season, in the tank on the SWS skid. Looks like the hotter it gets the more I will have to keep an eye on it. Luckily, my stuff is parked in a shop when not used, so that helps. Obviously, the goal is to be busy enough that this is not a concern. I would like to know if anyone has an answer to Gary's question above. Can you test this stuff? I'm sure you can, I just don't know how.