If I understand the question correctly, you're asking about the "minimum" amount of practice necessary to maintain skill, i.e., to avoid becomming "rusty"?
In our martial arts class, we recommend a minimum of three hours of hard training each week. More experienced students need more (up to 10 hours a week), and beginning students can get away with less. (Coincidentally, our classes meet twice a week, 1-1/2 hours each class...) This amount of training will usually result in a steady increase in skill for as long as the regimen is maintained. But, the range of practice needed varies GREATLY with the individual.
I've tried to adapt this concept to my pistol training. I'm still a beginner in pistol, so the regimen I've adopted is focused on basic pistol operation. For me, I typically equate 50 rounds of slow, well-aimed range firing with around 1 hour of training. I equate 100 dry-fires with around 1 hour of training. And, I will often train double-feed jam clearing with snap caps for one hour at a time. Do each of these, once a week, and there you go. You've got your three hours, and you will find that your basic pistol skills will improve steadily. (It helps with time management that the dry firing and the double-feed jam clearing can be done at home. You only need to go to the range once a week.)
Now, with respect to the advanced tactical training, if you're still a beginner like me, then you've got to add that to your 3 hours of basic training. Find what works for you. I haven't added any advanced tactical training to my regimen, because I don't believe that I'm that skilled yet.