My vetty says cerenia injections should last up to 3 days; however, Trix is on cerenia tablets every 36 hours and if she does reach a point when she needs an injection (like when she has a p-tities flare), I don't wait 3 days before restarting the tablets -I usually do it within 48 hours of the injection, because if I wait to long and she vomits again, the tablets won't stay down and we have to go for another injection...vicious cycle.
For her, I have found that cerenia (4mg tablet) every 36 hours works well to keep her settled. She has Stage 3-ish CKD coupled with some p-titis issues, so things can throw her tummy off at times. I get her tablets from ThrivingPets.com. The tablets are more affordable than injections.
Another thing I do with Trix, which may or may not work for you, is I pack all of her meds into an empty #3 gelcap, and they all go down in one fell swoop. There are days I curse the gelcap (when she repeatedly spits it out), but overall it is a blessing, because it prevents her from tasting the pills, and know they go down and she's not cheeking them. She gets 3-4 pills twice a day, all packed into one gelcap.