time scale for usa sibling visa for uk citizen

[mikeouk]

my sister has recently got american citizenship, we have filled out the I 130 form and will be send shortly.
my understanding is that there are a certain amount of sibling visas allocated each year for individual nationalities and for each region. Ive read that it can take 10 years, but is that a worst case scenario?
My sister is in miami so i would imagine there will be a big amount of Hispanic applications, but cant see there being many uk ones.
Idealy id like to emigrate in about 6 years, my son will be old enough to decide if he comes with me or stays in uk then (ive included his name on the application).
Are there any fellow brits gained us residency via sibling visa who can give me an idea how long it took them?
thanks