In a study done on women, who of course have far more estrogen in their blood, the researches found a huge reduction (85%) of serum estrogen levels within four days of administration.

http://www.ncbi.nlm.nih.gov/pubmed/1...?dopt=Abstract

As for switching over, letrozole has a half-life of 45 hours. IOW, the serum level will be cut in half about every two days. However, it also depends on the dosage as to how long the serum levels will drop low enough that letrozole is no longer effective. Even at a dose of 0.1 mg e.d. , letrozole will inhibit estrogen production.

http://erc.endocrinology-journals.or...nt/6/2/245.pdf

Those taking a dose of 2.5 mg e.d. are going to take longer to reduce their levels. William LLewellyn has an excellent book that addresses PCT. Probably going to take 2-3 weeks to get your serum levels of letrozole down to where it is no longer suppressing estrogen, but individual results will vary.

As an aside, the next time your gf/wife/mistress complains about your forgetfulness, show her this study about the effects of estrogen on the brain:

It appears that inhibition of estrogen synthesis in the brain may have beneficial effects on spatial memory.

http://cat.inist.fr/?aModele=afficheN&cpsidt=20016663

Be prepared to start dating your hand for the next two months.