How long could you stay in a crate? This is how you should think about it. The crate should be large enough for the dog to stand up, turn around and lie down and stretch out. Other than that, I believe a dog should not be crated longer than 2 hours. It is hard on their psyche.
In general, a puppy can hold its urine for one hour longer than its age in months, so a 3 month-old puppy could potentially be crated - for example at night, for up to 4 hours. I would only recommend this duration of crating at night and for puppies that people are trying to "housebreak."
If a puppy or dog needs to be crated longer than an hour or two, I recommend looking for alternative solutions such as day care, a pet sitter to let the dog out at lunchtime while the owner is at work, or an exercise pen with puppy pads during the day if the owner absolutely cannot be home.
Generally though I do not advise people who are working to get puppies because of the puppy's need for close supervision for safety, housebreaking and attendance to all the other needs of a puppy such as socialization.
Once a dog is housebroken a crate is not needed on a regular basis. Too many people crate their dogs all day while at work and then again at night. This is no life for a dog. I think it can be very damaging psychologically. But crating a dog for brief periods is fine. I recommend giving the dog a stuffed Kong or other safe chew toys while they are crated for diversion.