Hard, right now.
You can probably start laddering 1v1 now. Although vs AI (and 1v1 custom AI games) helps greatly in securing the basics (constantly building workers, managing production structures, mastering each race's macro perks), getting down timings (i.e. when the first barracks completes, how long warpgate takes to finish [with or without boosting], when larvae injects complete) and build orders, nothing is better than the ladder to hone those skills.
In addition, people in ladder can and will pull out the works (especially in gold [maybe plat, I haven't gotten there] and below). Things you won't ever see your enemy do in a vs AI game. Cannon rushes. 6-10 pools. Marine+SCV all-ins. Somewhat better players will use more refined, but still unconventional tactics that you won't expect. DTs. Banshees. Mass mutas (like half of every PvZ I play includes mutas, and I still suck at dealing with them). And so much more. The only way to learn how to deal with these is through raw experience, and ladder will give you that.