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I am a 26 years old guy who has been in the gym since he was 16 but doing it mildly and having long breaks because of moving to other countries and stuff.

Last year I moved to Netherlands and signed in in the gym inside the hospital where I work.
During the year I started with a strength training and then continued with a mass building program (both of these designed by the "instructors") and then finished with a self designed variation of the mass program in which I substituted some machine and barbell exercises with dumbbell ones, increased the reps, shortened the rest and increased the abs exercises in order to obtain some definition.

Now, I'll be back in the Netherlands on the 8th of August, I can start training again on the 9th, and my goal would be to build trainings with a few fixed characteristics:

a) short: they should last less than one hour
b) frequent: five days a week
c) dumbbell based: I feel that all my trainings have always been much more effective with these and also the gym organization makes it difficult to get the barbells when there are people doing spinning or bodypump, etc. There is though one problem about the dumbbells: they only go up to 20 Kgs. I am not there with any exercise, but could get there easily during this year, especially with Pecs and Arms.

My aim at least for the month of August would be to maintain the muscle mass and not accumulate fat. In September then I would change back to a muscle building program, or a strength one if we see I would need one.

My questions are quite straightforward: how many days? how should I subdivide the different muscle groups? Given that I would train at 5 pm everyday, what should my nutrition plan look like (I cannot cook in my office)? What supplements should I take?
How much cardio, and what should be my aim and program like about it?

As I told you I am not an absolute beginner and I would like to ear from you both because I saw that this forum is at a whole different level and also because I never analyzed these problems thoroughly and at the same time.

This is my first post here. I will soon provide a photo and data if needed (prefer to do it after my vacation here in Italy). Thanks in advance.