It's significant that these come from the first edition of Starting Strength, and they were omitted from later versions. They were a guide only, but people took them as gospel, or used them as an excuse for the humblebrag, "oh but I am elite on these but I'm not that strong."
More useful is looking at drug-tested raw world records. For example, here are the
women's, and here the
men's. Notice also there are age-matched records, with Masters 1 being in your 40s, and so on. My standards would then be a fraction of these world records
25% = minimum required for a decent quality of life and social-level sports
50% = good quality of life with spare muscle mass aside in case of illness or injury, and recreational sports requiring a lot of strength, such as climbing or rugby.
75% = around what you need to compete nationally in a truly strength sport such as powerlifting, weightlifting, throwing, etc.
and after that would just be pure barbell sports in most cases.
For example, a guy around 5'10" and 85kg or so, the squat world record is 303kg. If he's 30yo and previously sedentary, he'll probably start with about a 40kg squat. Most of us can agree that 13% of a world record isn't really enough to do anything. He gets up to 75-95, that's in the range of his current bodyweight, or his bodyweight after some months of running, or some months of lifting and bulking. And that's going to make the guy's day-to-day life a lot better, plus now he can play lunchtime soccer with less worry about popping a knee. He doesn't need to squat 180kg for his health.
50% is 150kg, in getting there he'll have built muscle mass so that even if gets sick or stops lifting for a bit, he can come back to it and he's not a complete newbie. And if he plays in any any social sport, and in some state level sports like volleyball, he'll be the strongest guy on the team.
75% is 227kg, and this is where he can start competing in barbell sports nationally, in powerlifting certainly, and maybe weightlifting if he's been doing the quick lifts at all. 75% of the WR total is 635kg. The drug-tested Powerlifting Australia record for the 93s is 727.5, which is 86% the world record, and the last-placed of 7 guys in the 93s at PA raw nats last year got 617.5, the winner 707.5. As for the untested GPC, currently they have 15 spots open for the men's 90s, and the 15th guy has 625, the 1st guy 787.5. So like I said, 75% of the world record is what you want to compete nationally in a barbell sport. It's also the sort of lifts you get from nationally-competitive throwers.
In other words it's like Dave Tate's informal standards, "shit, suck, good, great." 0-25%, 25-50%, 50-75%, and 75+%.
As for how long, well last night I was on the phone to a former client. He told me his lifts, and asked if that was where I thought he should be by now. I said, it depends. Time is less important than how many workouts you've done and how much you've pushed - and you may or may not be able to push hard. Do you work out twice a week or six times? Do you have a programme or do you "go by feel"? Are you 17 years old and still living at home while mum irons your undies, or are you 40yo doing occasional night shifts and with 3 kids? And so on.
I also see powerlifting competitions, and what I see is that over the years in each weight class you have one or two names that are always there, and then a bunch of others that are different every year. Again quoting Tate, if you have a crew of lifters, most will do one competition and then never anything again, and in two years if there is one other person on the crew with you that whole time, you are doing very well. Ten years? That's rare stuff. And it's the same in weightlifting. So this is where the standards are relevant, too - getting 25% WR doesn't take much out of you, hell some people can do it on day one in the gym, they're just big guys. 50% usually takes a solid effort over months or a year or so, and you do have to work hard to maintain it. 75% takes serious dedication and probably years of lifting, and not many people are willing and able to keep it up for long. 75+% is rare stuff indeed.
So I think broadly in terms of those standards, but when it comes to people I train, I just look for progress. On the one hand you have lots of guys going to the gym for years and just spinning their wheels. On the other you have 35 year old guys who try to do the Texas Method, you look at their logs and 12 months on their lifts are unchanged - rarely, they got injured, more commonly, they burned out. So I usually recommend a middle path, where you have easy, medium and hard fortnights, and then once or twice a year going really hard, and once or twice a year taking a break and maybe doing something else. Because I think nothing is as important as that you keep lifting. So I look for progress you can sustain.