For me it varies... I get it every 3 weeks.. and there have been a handful of times that I haven't felt my symptoms return before that 3 week mark, however usually around day 16ish I start feeling weakness creep in.. I wonder why, and then I do the math and realize it's getting near time for another infusion... I have also had times that I sware it only lasted one week!
When I talk to others at the infusion center - they all say it varies each time with them as well.. I guess it depends who your donors for each batch are, what your current stressors are, any illnesses, the temperature, hormones etc etc etc!

When I first started IVIG, I got it every 2 weeks.. and I always felt my symptoms returning around day 10...

Good luck!!!