While its certainly not a bad idea to TREAT your gelding as a stallion for 90 days, his sperm will decrease in a matter of days. "Extra" sperm is stored in the organ adjacent to the testicle, and both the organ (epididymis) and the testicle are removed as a single unit in the vast majority of castrations. The only sperm left in the stallion's body will be the very small amount left over in the vas deferens, and that will disipate very quickly. Dr Ed Squires did a project where they collected gelding's seminal fluids for several weeks after castration (until they failed to get an erection and mount) and the amount left over was negligable. Two weeks would be plenty, but more time apart from mares never hurt anyone, of course.
Two other factors will also affect semen quality:
1) Lack of testosterone. Testosterone is a required part of ejaculation. Without it, the gelding will not ejaculate. There is enough left over in the system for only a very short time. Blood levels will immediately drop and will be but a fraction of normal in a matter of hours, and will be quite low after the first week. Two weeks and it'll be pretty much lost.
2) Body temperature. The body is too warm for sperm to survive well, and they die off pretty quickly.
For some really interesting reading (if you are into reproductive topics like this one), check out Dr Squire's research paper detailing semen quality following castration. Its available from the Journal of Reproductive Fertility, Suppliment, 1979, pg 25-27. A brief summery:
"Twenty-three geldings were used to determine changes in seminal characteristics following castration and the effect of frequency of ejaculation on these seminal characteristics. In Exp. 1, semen was collected from 8 geldings every other day after castration until the number of spermatozoa per ejaculate was below 1% of the precastration value. An average of 3 ejaculates was required to reduce the number of spermatozoa below this level. In Exp. 2, 15 stallions were castrated and each stallion was assigned to 1 of 3 groups for seminal collection at 7, 14 or 21 days post-castration. The ejaculates collected on these days contained an average of 23, 14 and 2 X 10(6) spermatozoa/ejaculate, respectively. In both experiments, all spermatozoa in ejaculates collected 7 or 8 days after castration were non-motile. Frequency of ejaculation did not appear to hasten the disappearance of spermatozoa from the ejaculates. It is considered that after castration several months may be required before the ampulla and vas deferens become devoid of spermatozoa and the ejaculates azoospermic, and that pregnancy is unlikely to result from mating or insemination 1 week after castration."