it's possible to experience flatline after 2months, its possible to experience flatline from day 0, it's possible to experience flatline from day 0 to 3months, it depends, its linear, its bad tho, and mixed with withdrawal symptoms it's tough.
If what you mean is the lack of sexual desire, let me tell you that it is the most normal thing in the world. Sometimes it is normal not to feel sexual desire and it is not necessarily a problem.
Do not believe much in that of "the flat line". It is very speculative.