It takes about 2 months total.
Allow for c0ckups like the I-20 form going to the wrong address... U.S. schools are sometimes less than great when it comes to working out how to address a package to a "foreign" country. My I-20 got lost because they sent it to a UK instructor who was acting as their agent over here, and he was not around to sign for the package. Actually that caused me a lot of hassle because that school could not be used, and they resulted in a 'training not completed' entry on the US Immigration computer, and now anytime I enter the USA I get treated like a terrorist, with special interrogation.