I think it means 6 month to "fully modulate" your immune system. I've heard around 4 month btw
Half-life of interferon beta is quite short. It is normally, as you probably know, a reaction of your body to several viral infections. It's a cytokine, so it's tightly regulated. i can't imagine it stays for month. Maybe some side effects last.
Because of the short half life of interferons, interferon alpha for hepatitis c-therapy is PEGylated. As far as I know they were experiments for interferon beta as well. I don't know the results.
Intravenously interferon beta lasts only for a few hours. For subcutaneous injections the peak is after 1-8 hours, half life maybe 10 hours?? (my personal estimation). It should be cleared after around 60 hours, this is why you have to inject 3x/week.
I could ask some interferon guys from my institute, but we work with mice - and mice tell lies, at least they don't remind me of MS patients
But some people here measure interferon levels or reaction on interferon/viral infection with so-called reporter mice. It doesn't last long.