It depends on your promoter, age of the animal and which kind of Cre expression you would like to have. For stronger promoter like CAG in CNS, 1 week is enough.

We are delivering 3 mg tamoxifen per animal once daily for 5 days in the week before our experiments begin. We are now running an experiment where we need tamoxifen available in the animals for 14-28 days, but it has been shown that tamoxifen is only available in the brain for 8 days after a similar dosing run. Do you have any insight in a long term dosing interval to keep tamoxifen available through our experiment in the brain?

Once you induced Cre, most of case it will be stable, so I am not sure why you need tamoxifen in there for 28 days? Your dose should be sufficient to drive any Cre expression.

The attached paper shows that tamoxifen degrades in the brain after 8 days. Therefore, we are concerned about the bioavailability of tamoxifen in the brain after 8 days. Any insight?