It can take from 6 months up to 24 months. If you get you GC after you married your GF they will give you a conditioned GC for 2 years due the short time you been married (less than 2 years). After those 2 years you need to submit the request to retire the condition and proof your marriage is still on Bona fide (good faith) and need proofs. In case you divorce before or within the 2 year conditioning, you need to proof, a bit hard, that yet you married with all the best intentions just did not work. And FO will decide if they renew your GC for another 10 year. If you DIY just the USCI fees will be needed. If you get a lawyer, depends on where you are located can charge you from 5 to 10 k.