No specific levels here, but a Pokemon can only have up to 510 EVs at one time. This means that a Pokemon can have 255 EVs in 2 stats each, but people stick with 252 EVs in one stat because a stat goes up by 1 point at level 100 every 4 EVs. There are some cases when 252 EVs in a stat isn't necessary, however.
If you're still not sure, read this