How many years of annual income history do I need to sponsor my spouse?

You need to present the most recent tax return and information about earnings for the past 3 most recent years.

Significantly more information is needed in order to analyze and advise about immigration eligibilities, options and strategies. Are you a U.S. citizen? Is your fiance located in the U.S.? If so, what visa(s) has your fiance had? Etc. Regarding the sponsoring spouse's income, however, the answer is more straightforward. You would need to prove your current income, and while tax returns for previous years can be important, they are not lawfully necessary. To prove current income from your current job, generally you should plan to supply a copy of your employment contract, an employment verification letter signed by your employer (identifying such things as your job title, date of hiring, rate of compensation and expectation of continued employment), and copies of your most recent paystubs. It also could be helpful to supply copies of recent bank statements showing actual deposits of your earnings into your account. If your soon-to-be spouse is in valid nonimmigrant visa status at the time the couple files an adjustment of status suite of applications, your spouse could expect to receive Advance Parole (a "travel document") within 90 days of filing, and that would allow travel abroad and re-entry into the U.S. while the adjustment of status application remains pending. It would be wise for you and your fiance to confer with an immigration attorney who, after learning all of the relevant information, could answer your specific questions, advise about the steps to take and offer legal representation for the often complex application process. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.