My Gleason score is 7 - 3+4. How long can I live without treatment?

A few thoughts to offer. It sounds as if you were given a standard systematic 12 core biopsy, which takes 12 samples from different zones of your prostate. I would think that having 2 of those cores positive with cancer indicates a fairly sizeable lesion or more than one lesion. What is your PSA? Have you had a 3T MRI. The MRI will shed much better light on the size and location of your tumor or if there is more than one lesion. A genetic test of the tumor tissue such as Decipher or Oncotype dx would shed light on how aggressive the Gleason 4 cells might be. Your urologist could and should be ordering these kinds of tests for you and advocate for yourself that they be done, if this isn't the case. This kind of additional information will go a long way in helping you make a better informed decision on what to do for your particular case.

The problem with a Gleason score..."any" Gleason score...but especially a lower/moderate score, is that you really know nothing about your cancer. I am a Gleason 3 + 4 = 7, that at the time of biopsy and in the post-radical prostatectomy showed only 6-10% "4" cells. I was barely a 3 + 4 = 7...as close as I could be to a 3 + 3 =6. But...and that is a big "BUT"...my urologist said flat-out: "I'm taking your prostate...there is no logic or purpose to 'wait and monitor' with active surveillance, because all you are doing is waiting for the inevitable of the cancer progressing to something worse", to include everything I will describe below:
My urologist/surgeon and I went into surgery confident that we had "caught the cancer early", and we did in one sense, but...the Gleason score says nothing about the microscopic cellularity and molecular level data. That is why getting the "Decipher Test" at the time of biopsy is important. I was not told back then that my insurance company would not pay for the Decipher Test. I was not given the information so I could decide to pay cash for the test. Well, everything changed after surgery in the surgical pathology report:
I had Extraprostatic Extension (EPE) of the tumor. I had "Surgical Margins" meaning the physician didn't get all of the tumor tissue/cells, out of me; I had Cribriform glands (ominous issue of cells in sheets that look like Swiss Cheese as one author describes it); I also had left seminal vesicle invasion which instantly changed me to be a pT3b category, despite my Class/Grade "2" and Gleason Score of 3 + 4 =7, again with just 6-10% of cells being grade "4." The invasion into the seminal vesicle makes you the pT3b and lowers your longevity from 10-15 years or more, to more like 5-10 years "if" you are properly treated. A pT3b cancer just "always comes back"...reoccurs ... within the first 5 years. It could happen 6 months to a year later, or might take up to the full five years. Either way, the vast majority of pT3b category cancer reoccur.
So, that was a long story telling you that a Gleason Score really tells you nothing. It is more valuable the lower the score is, telling you that you likely do not have cancer, or if you do, it may very well be early in its stages, and easily treated with good outcomes. It seems though that once you are at a "moderate" risk 3 + 4 = 7, or worse...a 4 + 3 = 7, it is a crap shoot. You really do not know, as I didn't, how severe and aggressive your cancer is, until your prostate is removed and you get your surgical pathology report that gives you...or spares you...the bad news. Get that Decipher Test done. It can still be done on your biopsy tissue. You'll have a better chance of knowing how aggressive your concern is or isn't.