How long does it generally take to gain a Skillcape in any particular skill

This always varies, from person to person, skill to skill. Some skills, like Runecrafting, don't always take that long, but can take 15 minutes per load, and other skills, like Woodcutting, can take a long time when cutting higher-level trees, and not alot of time when cutting lower-level trees. In other words, it is impossible to predict exactly how long it would take for you to gain 99 in any skill, but, it will not happen quickly, if you play regularly, it could possibly be achieved in two months, under if you really do play alot, but, if you just play in from time to time, and don't go on that often, it might take about 6 months to a year.