Very Little Change After Botox on Forehead

Normally at this point, most patients would see noticeable results after a Botox treatment. If you can still raise your eyebrows, then you may possibly be able to have more Botox injected to help soften those lines. If the lines are present, but you are not able to raise your eyebrows at all, then more Botox will not improve your result - it may actually cause ptosis. Botox does not always eradicate lines. It can soften them and prevent them from worsening over time with repeated treatments. It is best to consult with the person who performed your injections to be evaluated in order to determine the right course of action for you. 

Botox works by irreversibly inhibiting Acetylcholine release at the neural synapse. It takes 3-5 days for the remaining Acetylcholine stores to be used up, and as they are used up, the Botox will take effect. Waiting additional days will not enhance the results.
If you feel that you did not see results, have a frank discussion with your plastic surgeon about the administered Botox and determine if you need additional units of the Botulinum toxin.

Dear Annie,
As the post above attest, there could be MANY reasons why your Botox treatment may have been a disappointment to you.
By 12 days you would have seen a complete effect of the Botox. So - the period of elapsed time is not the answer.
Nor do I believe that your surgeon would have partially treated your frontalis muscles generated wrinkles without telling you as some of the above posts was suggesting. Yes -- weakening the frontalis in people who need it to prevent upper eyelid skin from dropping lower is counterindicated BUT a surgeon would tell you why they chose not to treat those folds rather than allowing you to believe they did.
Botox is most effective in smoothing active wrinkles - the wrinkles that appear on facial animation. You plainly state that "the lines were already visible at rest". The likelihood of Botox erasing established lines is very low. To do so would require Botox AND a filler (Restylane/Perlane OR Juvederm).
Botox efficacy depends on how it was reconstituted (gently or allowed to bubble which reduces its potency), was it over diluted (ie you are getting less Botox units than you should) and was it old and less potent. Always possible when a surgeon is not busy and reconstituted Botox sits in his/her fridge.
Lastly, Botox injectors are NOT the same. Some surgeons are better at doing it than others. Witness if you will the plethora of freakish botoxed celebrities with the "Spock look" (depression of the medial brow with unnatural elevation of the lateral brow). I am sure they paid a lot for their treatments -- but such Botox results are far from optimal.
Hope that helped.