My personal opinion is that is a "rule of thumb". All dogs are different and you can & should adjust accordingly. It will also depend on the activity and what's going on all day. On a workday, when he's crated most of the day, he may need a little more before and after work. On your day off, when he's out and about he may not need as much.
I think the 5 minute per month of age rule is out there to help people guage what is appropriate for a pup so they don't take their 3 month old pup for a 2 hour hike or 5 mile run. It's not a hard and fast rule. Miles is 5 months old and he gets approximately 30 minutes 2x per day and he's got plenty of energy left over. Sometimes we go longer. For example, last night we went for a walk in the woods and swimming. He did some water retrieving, and running around in the woods and I'd say we were out there a total of 45 minutes. That included walking, running, sniffing, pooping, swimming, retrieving... He still had plenty left when we got home. Sometimes, when we are off work and we have people over who bring other dogs, he plays with them for hours and we skip the morning walk the next day entirely.
Anyway, I'd say it's a guage but use your judgement.
The rule of thumb is not set in stone and yes, you should adjust accordingly depending on the individual dog. To be absolutely honest I had never heard of the 5 minute rule before I came to this forum. I was well aware of the dangers of over exercising but the rule was new to me. The rule makes sense, however I don't think it will damage your dog by going over the recommended amount now and then.
When my dogs were allowed out and about for walks (12 weeks +) we went around one of the fields at the end of my road once a day. This took 20-30 minutes. They were off leash and free to explore. I don't believe I caused them any damage by doing this once a day. At 12 months I was doing 1 1/2 - 2 hours of exercise a day with them, give or take. All off leash and non-forced.
At 19 weeks, Callie pretty much let us know when she was tired.....she would just stop and lay down for a minute or head off for a drink and a laydown...this was our cue to go ahead and get back on into the house because she could usually get a second wind going if we waited too long.
Each pup is different.....just use your own sense of what is enough for your pup judging both by her actions during the exercise and after bringing her/him back into the house. If we ever misjudged and thought she had had enough but her behavior upon returning inside showed that she needed a bit more release, we simply played ball on the kitchen floor for a few minutes more...of course she's too big for this now, but she's also learned house rules (7 months tomorrow) so can control herself until the next trip outside.
I'm home in the mornings, so Lexie is just crated for 4-5 hours in the afternoon typically.
I walk her in the morning for 20-30 minutes, some mornings Rob also takes her for a short walk right before work. When we get home we go for a walk; this is the longer walk usually about 45-60 minutes. Since the weather has been mild and she finished all of her shots a couple of weeks ago I've just started taking her to the dog park (which she loves!), but we usually just go for 30-45 minutes (I can tell when she's starting to tucker). We still do our two walks when we go to the park or have other big outings. I've found that even if she seems really pooped after an afternoon or evening at the park or at the water she will get hyper again around 9:30 p.m. so the walk is a must every night.
She's still not really into retrieving yet, we keep working on it, but she's not getting much exercise in that way (I wish she would). She loves to do water retrieves, but we're not close enough to water to make it an evening activity.
I just get concerned because puppies will go, go, go and I don't want to let her overdue the exercise at this point in her growth.
That just seems kind of a long walk for a 12 week old puppy. I think the best rule is the 'no forced' excercise. If you walk your pup it will walk with you. If you go into a field and walk a bit but then sit down or rest often your pup will explore on its own but also feel like it can lie down or sit down and relax.
OK, so 19 weeks is just about 5 months of age...I was taking Grace at that age to obedience once a week for an hour and handling class for an hour, and other than that, she just got what ever excercise she did on her own in the backyard, maybe a leisurely walk through the neighborhood. Since we have 2 other dogs, there was running, jumping, and wrestling that she did when she was playing with them.
I wouldn't over think the whole thing. I wouldn't take the dog to the woods for an all day hike or go jogging with them, but you can take them places and for walks and stuff like that.