Hilchos Bikkurim 10:15

Optimally, one should put the first shearings aside for the kohein, but if one did so in the middle or at the end, he has fulfilled his obligation. Let’s say that someone has five sheep but he doesn’t shear them all together. Rather, he shears one and sells the wool, then the next, and the next, etc. In such a case, all these combine to obligate him to give first shearings. This is so even if this occurs over several years. One may take new shearings for old shearings, and from one type of sheep for another. However, let’s say that a person has one sheep. He shears it and puts the shearings aside, then he buys a second sheep and puts its shearings aside, etc., until he has shorn five sheep. In such a case, the shearings do not combine.

Hilchos Bikkurim 10:16

If someone has a number of shearings and he wants to divide them among several kohanim, he shouldn’t give any kohein less than five sela in weight of white wool (a little more than three ounces*), which is enough to make a small garment. This doesn’t mean that he must bleach it before giving it to them. Rather, he should give each kohein enough unprocessed wool to result in a minimum of five sela of wool after whitening. This is inferred from the phrase “give it to him” in Deuteronomy 18:14, which we take to mean a significant gift.

*[For context, this is about 250 yards of yarn. That’s enough to make a ski cap or a pair of gloves – in other words, “a small garment.”]