Opened pectin will last about a month in the fridge, in a sealed container. Mold will be obvious, when it finally has to be discarded.
I seldom use any glue on a cigar. When I need to make up a cigar in advance, I just twist and knot a pigtail in the wrapper at the head.
When I do use pectin, I always start with a teaspoon of powdered pectin, and add tiny amounts of water to it, in a close-able cup, aiming to make the thickest gel possible. I spread it lightly on the terminal (head) edge of the wrapper with a fingertip, then finish with a tight pigtail or pigtail knot. The more viscous the glue, and the less of it you apply, the faster it dries.
In conjunction with a pigtail, the only advantage I've seen with using any glue at all is that, after cutting the head in preparation for smoking the cigar, you can wag the cigar in your friend's face, without it coming unwrapped. If you just cut it and stick it in your mouth, there is simply no need for glue.
I've rolled about 3000 cigars that I'll admit to. Fewer than 100 were glued. Glue is for factories and store display cigars.
Bob