Carlos, good question. None of my leagues has any specific times in the by-laws or elsewhere. If I were waiting on a bowler who was very slow, I might tell the bowler that I will bowl first.

I'm pretty sure there is no rule. 

Here are the rules in pdf: 
http://usbcongress.http.internapcdn.net/usbcongress/bowl/rulebook/USBC2013_14Rulebook.pdf

It is listed in the etiquette section:  Etiquette Information

A few simple guidelines to help make your bowling experience more pleasurable:
• Be ready to bowl when the pins are set, but wait until the pinsetting machine has completed its cycle and the sweepbar is raised to make your delivery. Throwing a ball before the machine is ready may damage the ball or equipment.
• Observe one-lane courtesy. The bowler on the right should bowl if both reach the approach at the same time. Stay off the approach while you wait.
• Taking too long to aim or get set up when you are on the approach delays the game.
• Try to remain in your approach area while delivering the ball.
• Step off the approach once you have delivered the ball.
• Good bowling requires concentration. Have fun, but keep in mind excessive movement or noise could be distracting when others are up to bowl.
• Excessive “lofting” hurts your game and damages the lane.
• Get permission to use another player’s ball.
• Refrain from using abusive language or obscene gestures.
• Play the game to win, but be a gracious loser. Good sportsmanship is always the key to a successful game.
• Wet shoes are a safety hazard. Watch where you are walking.

No specific USBC rule.  Surprised more leagues don't make a rule about being ready to bowl.  I have never bowled a league where there aren't at least a couple of guys who are never ready.