This will vary, depending on the harshness of the cycle. If you are taking more than two prohormones at a time, you will want a stronger dose of a SERM compared to a milder cycle or if you are taking a non methylated prohormone.
Below is an example guideline for a normal cycle, followed by an example for a stronger cycle:
As you can see, a SERM depending on the strength of your cycle will be stronger to combat the effects of hormonal levels and then are slowly decreased over 4 weeks. You will typically take a SERM for the same length of time you spent on cycle.