Guess I would look at the circumstances. General rule of thumb is that the puppy can hold his/her pee/poo 1 hr. for each month he/she is old. IMO that would apply to being crated/ex-pen or sleep time. When there is excitement (i.e. car-ride, playtime) that rule bites the dust, so to speak. Then there's a chance of piddle as soon as 15 minutes. Again this is only IMO.

I've actually rode with a piddle pad on my lap when my furbabies were young. (Now I don't like to take them with me because I haven't bought car-seats yet.)
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