As I had to use my AMEX for all purchases, and as it is a charge card, and I have no idea how high my limit would be, I am not able to limit myself to 20-30% of the limit (the way it is frequently recommended).
Almost every version of the FICO credit scoring algorithm in use today does NOT factor a charge card into utilization. Only actual credit cards and other loans. (So it doesn't really matter what you're putting on that card.)
So with this information, what would be your estimation on how long it will take me to build up a credit history that allows me to participate in all the stuff that you guys are talking about every day (Chase UR and other special sign-up offers...)?
You could very well already be there. The main obstacle is having
some kind of credit report. That Amex card should be reporting already and that might be enough.
I can recall people talking about 1-2 years but, really, the only way to find out is to try.