I have done tune ups that I think were proactive cause I didn't really feel "sick", just sluggish... and sometimes it was only after a couple percentages drop in FEV1... So, is this proactive? I'm not sure. In those cases, I usually do 14-16 days. I only stop once my FEV1 is back to baseline or higher AND I feel amazing. Like no cough at all, tons more energy that I never knew I was lacking, etc.
I think the studies at the U of M (and was it Denmark?) were done on proative tune ups and they never lasted longer than 2 weeks no matter what. I thought the idea was that since you're just kicking back the bug growth, but not treating an active exacerbation, you shouldn't go too long or it may breed resistance to the meds... Someone correct me if I'm wrong??
In cases when I FEEL I need a tune up, I've been on IV meds for up to 5 1/2 weeks. That was the last time cause they had me on the wrong med to treat what I was growing for about 10 days. Urgh. So, it started after that. Which would mean it was still over 3 weeks. Again, I wait until I have NO colored sputum or very little cough at all and when my FEV1 is at baseline or above.
Since you can't go much by FEV1 right now, you could always ask to keep the PICC in for a couple weeks and see what happens. Don't have them continue meds, but have them let you keep the PICC "just in case". My docs have suggested that to me in the past when they want to stop meds and I want to continue (I usually talk them into continuing cause I feel I know my body and the signs I am better.) Its kinda a "middle ground" though to keep the picc for a while longer in case your FEV1 isn't where you want it. Just an idea.
Best of luck!