My first cycle was deca only and i got decent gains. I did 300mg/wk for 10 weeks and i gained 17lbs and kept 13 of it. I would suggest that you add some test in there so that you don't ahve to worry about any kind of deca dick.
200mg per week for 4 weeks (350mg in first week). Given long action I was still gaining in weeks 5-6. Rehabilited a nasty shoulder injury too which was why I used it in the first place. Gained 18 lbs. and kept 14 after 6 months off (had knee injury after that so had to stop training for a time - still not too much loss). This was a bad year for me injury wise.
No deca dick. Test levels returned to normal about 3 weeks after deca wore off (14 days from last shot). No clomid or hcg. No other sides and I felt fine.
I hadn't used drugs in the 2 years preceding this cycle and I'm fairly sensitive to AAS (I make good gains on 100 dbols over 4 weeks too). I train all of the powerlifts as well as olympic style movements (no competition now so I could give a shit about weight gain). So keep that in mind.
My advice would be to not extend it beyond 8 weeks.
It will still be active in the system up to 3 weeks after the last injection so test suppression is a concern at this point.
You also don't see the same dramatic gains. After 8-9 weeks it tends to taper off and it's not worth continuing at the low dosages. Getting the body back to normal is a priority.
I DID 2 CYCLES OF DECCA ALONE.I DID 8 WEEKS OF 300MGS A WEEK. THE FIRST ONE I GAINED 18LBS AND KEPT 15 OF IT. DIDN'T BOTHER TEST LEVELS AT ALL. BUT THE SECOND TIME AROUND DIDNT GAIN SHIT BUT WAS HARD AS A ROCK. AND HAD DECCA DICK THAT TIME. FUCKED MY HEAD ALL UP TOO THE SECOND TIME. I DONT THINK ILL DO THAT EVER AGAIN. ALWAYS STACK IT WITH SOMETHING.