"It's about time <pronoun> <verb>" is an idiomatic usage in English, you won't manage to make logical sense out of the preposition here.
You could say it means 'almost time..', but even then that's slightly different from the intended expression, which means 'let's leave now'
Both of these are set expressions, and you can't really separate the prepositions from the nouns that follow them. That is, we're not talking here about "about" <<...>> but about about time<<...>>.
About timecan mean one of two things, depending on the context.
In the context above, it means that it's the proper time, but not necessarily exactly to the minute. You might say it means "around the proper time, and not much later".
You might also say, after waiting for a friend who was to meet you for lunch at a particular hour and finally arrives ten minutes late: "It's about time! I was worried!" Here "about time" really means "It's past the time!"