Depends on how much Effexor you are prescribed (dose), and the half-life. If you take an SSRI, you need to stop taking it for at least a few days before the roll in order to feel the full effects.
Effexor is not an SSRI, because by definition SSRIS (selective serotonin reuptake inhibitors) ONLY inhibit the reuptake of serotonin. Effexor also inhibits the reuptake of norepinephrine, which SSRIs do not significantly do. Thus, Effexor is an SNRI (serotonin norepinephrine reuptake inhibitor) and has properties of SSRIs as well as its own norepinephrine properties.
Because Effexor strongly inhibits the reuptake of serotonin, just like SSRIs do, it is not possible to roll on Effexor.
However, some people find that the withdrawal effects from SSRI's are too horrible and not worth abstaining from just for a roll. Some people can get away with it, but others become very anxious, panicked, dizzy, light-headed, etc, from SSRI withdrawal.