just let it grow as much as it will before getting into that between full and not stage then shave it and start again. i rock a full face stubble pretty much 90% of the year but when it starts getting scragly longish i buzz it back down to size and every month or so i give it a clean shave to refresh and i've noticed that after doing this for about a year when I do let it grow back in wild it's much thicker and grows much faster. I can now go from clean shave to about a centimeter in length in about 5 days, and I get a nice stubble the morning after a shave day.