The question I ask myself is, "How much is this mistake likely to bother me?"
"frogging." I hate the term, I find it saccharine and annoyingly "dweebish mom humor," but what are you gonna do? That is what everyone calls it.)
How do you know when it's time to frog? Look at your mistake. How big is it, really? Be honest. If your plan involves "blocking it out," be aware that this is not likely to work unless your mistake is off by less than an inch. If your plan involves turning one side away, or hoping no one notices, these are warning signs.
The question I ask myself is, "How much is this mistake likely to bother me?" Imagine yourself three years from now. You have forgotten about what it was like to rip it and re-knit it. All you have is the finished knit before you. Is your future self bothered by the mistake?
Sometimes the answer is yes. Sometimes no. Frankly, when in doubt, I advocate ripping it back and re-knitting it. You pretty much can't go wrong, even though it stings a little to contemplate it. But you like knitting, right? That's why you do it in the first place. Well, this is just more of what you love: knitting. How great is that?
Please don't hit me.
In both recent cases, the problem affected the fit so drastically that I had no choice. It was either "rip it back and re-knit it" or "never be able to wear it." And in both cases it was a gauge problem. Surprise!
The first time was totally my fault. It was a sock yarn I hadn't used before (Red Heart "Heart and Sole") and I didn't swatch. I just figured it was "pretty close" to the sock yarns I had used recently. Well, it turns out that wasn't the case. I knit from the toe to halfway up the calf before I thought to try it on. Uh oh!
In the second case it was a cabled cowl that is supposed to fit over your head like a hood. I swatched in stockinette and got the specified gauge. Which makes it a mystery that once I started knitting in the cable pattern, the gauge was off by a factor of about 40%. Ouch.
Frogging is just part of what it means to be a knitter, I'm afraid. Sometimes you just have to grab a glass of your beverage of choice and get to it!