You may also consider the pH of the mash. The enzymes that convert starch to sugars have a pH range they operate best within. I realize this is your first brew but it is something to consider as you progress.
As you progress you will begin to see trends and then can make adjustments. It could be that given your water, mill setting, equipment, processes, etc yield a slightly less efficient output than the brewer who wrote the recipe you’ve used. When you do have some consistent results under your belt you can make confident adjustments to meet your expectations. I rarely use recipes as written to meet a certain OG. There’s usually a slight increase/decrease to reach their spec when using my brewery.
As far a mash time:
Over a series of half a dozen + brews days, using the same ~12# grist, milled at the same ~.040 gap, 1.75:1 water ratio, 5.5 gal strike, 3 gal batch sparge volumes, mineral additions, recirculation pump flow setting, etc, I took samples at 20 minute intervals throughout the entire mash. I noticed very little pH change over the entire mash ...like +/- .03 cooled to room temp (actual data from one of those brewdays below), which is well within the margin of error given the MW101 pH meter and loose nut between the chair and meter.
When graphed, the data from that series overlaid upon each other are nearly indistinguishable one from another. Very consistent results.
Over that series, I noticed most of the SG increase takes place within 20 min. @152*F. ...but I measured additional SG increase to about the 100 min mark. Past the 100 min mark the SG did rise but very very little.
20 min 1.045, 5.47 pH, 65% of OG
40 min 1.056, 5.47 pH, + 11 points, 81% of OG (+16%)
60 min 1.060, 5.50 pH, + 4 points, 86% of OG (+5%)
80 min 1.065, 5.44 pH, + 5 points, 94% of OG (+8%)
90 min 1.069, 5.46 pH, + 4 points, (+6%)
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