Once or twice a day should be perfectly safe. There are two aspects to VED use:
1. Getting a full erection while in the tube with vacuum still applied. If a bit lacking at the moment, this aspect should improve with regular use.
2. Keeping the erection long enough for intercourse. This is where you will need to adjust the amount of constriction -- enough to keep it firm for long enough, but not so much that it hurts or turns purple and falls off
I use office rubber bands with my cheapie pump -- you may find they give you more variable adjustment.
As with any new exercise it takes a bit of practice. I will mention that my wife hates erections produced with the VED. I won't go into reasons -- I got censored last time. So I use the pump only for exercise, and rely on injections for the real thing.
Age 63. Other than cancer, in good health; BMI 20
Pre-op: No symptoms; PSA 5.7; Gleason 4+5=9; cancer in 4 of 12 cores
7 March 2008, RRP, non nerve sparing
Two nights in hospital; catheter and staples out after 7 days
Continent, no pads needed from the get-go
Post Op: Stage pT2 M- N-; clear margins and lymph nodes; Gleason 4+4=8; prostate weight: 37gm
6-week and 7-month PSAs: 0
Bimix injections working well