Tanya, 17 days is allot of 5-FU, but many don't see real "erosion" until into the third week or day 21 plus, so you will know in 5 days or so. I did 6 weeks of tretinoin (Retin-A) as advised by my doc, so top layers of skin already cleared up when started the flourouricil. For me on day 21 all hell broke loose and eruptions, burning, oozing - you name it - began. Up until then just red areas and some stinging, so came on quickly when cells started dying off. You may not have the same, but again you need to go another week or so. Just call you doc or their nurse for a consult over the phone. Most treatments on arms and legs are for four weeks/28 days.
Good luck! BTW, you had asked me a week ago how the arms were healing, and finally the scabs closed up 4 days ago. No more grease and pain lotions at all now! I'll post this weekend, that will be 10 weeks. Whitey