Deca-Cycle is 10 days.
Orbital-Cycle is one month.
Solar-Cycles is a day.
1 Mega-Cycle is roughly an hour.
1 Stellar-Cycle is a month.
A Nano-Cycle is 1 second.
Stellar-Cycle is 1 year.
These are the units of time I have used when Galvatron and the Decepticons go to Earth. Then, I imagine they'll use Earth measurements of time.
If you were to look through the TF Wiki, there are a lot of contradictions due to different version's of Transformers using or making their own. In my story, I've tried to streamline it to the best of my ability. I hope this has helped.