According to the website steroidplotter, if you pin 250mg of Deca once per week for 5 weeks, you’ll need about 114 days on average to clear all the mg out entirely. So while the graph shows the effective blast phase is probably ~7 weeks when taking 5 weekly shots, you will have solid HPTA interference for about 4 months.
Deca is not a good compound choice for someone cycling off completely for this reason. Someone who is both intent on restarting natural production between blasts and using Nandrolone should choose NPP, but I think 19-Nors in general are still much tougher to recover from than test & DHT derivative compounds.