Getting your GED requires taking placement tests to see where you’re at and what you know as far as math, grammar (English), and some basic science. Nothing very hard, just making sure you can function and have a base level of knowledge.
Once you’ve taken the placement tests, you can begin classes. I know some places it costs a lot of money, some places it’s not that much, and some places it’s mostly free. Where I live, the placement tests cost money (not sure how much) and the classes are free.
Classes in my town are 3 days a week (Mon/Tue/Wed), for 2 hours. There’s 3 sessions a day. At 10am, 12pm, and 6pm. How long you need to take classes depends entirely on where you start from, how often you decide to attend classes, and how hard you work/smart you are.
In the US, we have 12 years of mandatory school. The last 4 (9-12) are our high school. So say you have two people, one who completed 9th grade, and the other who completed 11th grade. The one who finished 11th grade will most likely get his GED faster, since he has less to learn. GED classes are not mandatory, so some people don’t show up very often (I’ve helped teach GED classes before), which is silly, because they take longer to learn the necessary material. And if you never practice math or reading at home, or just aren’t very smart, it’ll take longer to get your GED.
Anyway, I’d say you could do it in 6-12 months. Depends largely on how good your English is. You’re Swedish? Your English looks decent from your posts. I’ve taught many Hispanics and Africans who know basically zero English, learn the language fairly well within 1-2 years, and get their GED within another year. Part of this depends on their work ethic - most of the people I’ve taught work 60+ hours a week and are very driven, but some don’t try very hard and make very little progress.
So I’d say you’re looking at around a year to fine tune your English and get your GED.
Like @SkyzykS said, is this something you really want to do? Do you want to uproot your entire life, lose the business you own, leave your family, to become a soldier in a country you most likely have no specific care for? Another thing, if you join the FFL, is that like @zecarlo said, you’ll have to learn French. I’m sure they have requirements about the minumum amount of education required, much like the US’s GED program, and you may end up taking even longer than you would in America, depending on if you know French (do you?) or not.
All in all man, I’d say it may not be worth it. Does Sweden have some type of program similar to the US’s GED? If so, you could complete it while still working full time. I know many people who are older with children who have done it. A young, hardworking man certainly should be able to, if he has access to it. Or do they have some type of thing like the National Guard? Where you’re not really a “soldier” as in you get deployed but you do training monthly and stuff like that?