There isn't a standard for that. Different brands will come in different sizes.
Say a pattern says to use 2 balls Lionbrand Organic Cotton. If it doesn't tell you how many yards are in a ball of that yarn, you may have to go somewhere else and look up that info. You may find that one ball of that specific yarn has 65 yards. So you know you need 130 yds of yarn for the pattern. Say you want to use a different brand of yarn that will give you the same guage but it comes in a ball of 50 yds - you know you'd need 3 balls of that yarn.
Mirl56 at Ravelry, too.
Learn from the mistakes of others. You canít live long enough to make them all yourself. Eleanor Roosevelt
Lady Feb Sweater
more shawlettes, socks