My husband, also a Ta stage cancer, has been in a trial that includes two years of maintenance Gem/doce infusions. As I usually handle the scheduling I thought I would answer your question. Twice during the two years we have gone out of the country for a month. Both times we adjusted the infusions to accommodate the travel allowing a travel day and a recovery day before or after the infusion. They let us slide the infusion by up to 11 days but we never skipped one. That means there might have been 5 weeks between them instead of 4. We did schedule the infusions carefully over this entire two year period to squeeze them in. (the last one is scheduled for this June). He also had cystoscopy every three months.
It was a challenge but doable thanks to the medical team and scheduling staff.