You apply at the beginning of your sophomore year, if that’s what you’re asking, and you will have to have junior standing by the term you would be starting (at least 60 units). For example, lets say I’m taking 15 units per semester, or 30 units per academic year. At the end of my freshman year, I’ll have 30 units, and I’ll apply to Cal in the beginning of my sophomore year (November, I believe?). If I get accepted, then I would transfer to Cal with junior standing.
Summer/winter classes offset the amount of units that you’ll have to take. The 60 unit minimum is actually fairly easy to reach, one way or another. If you have units from AP tests, that would satisfy courses and probably give you units, as well.