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This isn't to extraordinary a problem and some dogs can be very funny and particular in their toileting habits. There is only so long that a dog can voluntarily hold on to their urine without the body over-riding this desire, forcing a urination. So if your dog really needed to go, then they would regardless as to whether someone is there or not. It would only really be from a clinical problem affecting the bladder or nerves of the bladder that could cause an inability to urinate and as such cause the complications associated with a blocked bladder.
So I wouldn't worry and continue to do what you are doing.
It is unusual to have a dog specifically trained to urinate only in the presence of another person. However, this length of time without someone being present shouldn't be a problem as if there is an absolute need to urinate, the dog's willpower/training couldn't overcome the bodys need to void urine. It isn't an ideal situation as retained volumes of urine can pose a higher chance of urinary tract infection. But the dog would unconsciously urinate before any issues arrive.
As far as house training a new dog. It wouldn't be of any benefit of advantage to train a dog to urinate only in the presence of a human. I would concentrate on it learning to urinate outside in the garden or on a walk rather than indoors.
Hope that makes sense
In an ideal world, I think it would be better for the dog to learn to urinate without the need for someone to be present. However, if this behaviour is already be learned, then it may be difficult to train out of it. Ideally, urinating outside when the dog chooses would be better.
If this behaviour continues (urinating only when someone is present) I cannot see any significant issues with it carrying on. The dog will not hold on willingly to urine until you are present. Urinary tract infections can occur more readily following periods of urine retention, but this is not a sure thing. . Allowing the dog to urinate outside without humans will simply allow for an easier routine all round (for dog and owner) reduce the periods or urine stasis (reducing the possibility of urinary tract infections) and most importantly, it allows the dog to exhibit a normal behaviour of scent marking whilst outside and urinating whilst exercised.