I guess it depends on how massive your own plant is. I like to let my plants get established before dividing them. I know that some people have Utricularia plants that grew from a tiny bit of stolon (I grew U. sandersonii from a 2 mm bit that the postal workers managed to miss when they did whatever they did to the package), but this all takes time, sometimes more than a season. Larger starts grow better, and establish more quickly. For aquatics, I think a tablespoon full of plant (wadded up) is a good minimal start. The aquatics often have a period of adjustment, and there is some stolon reduction, and there needs be enough for this to happen. If your plant can spare this much, then go for it. If you mean what season is best to take the cutting, I would say probably summer when the growth is most rampant, but really you can do this at anytime you have sufficient material to spare. Once U. gibba becomes established it is a very strong and rapid grower.