Basically, just as soon as the IRS approves the electronic version of the forms that were changed to accommodate this. It could be awhile too, because the IRS is already scrambling to update their own computer systems to deal with this, and other changes that occurred in the middle of the tax filing season.
Since the spending bill with the tax extenders has just been passed and signed into law, both the IRS and tax preparation software companies have to change various forms, schedules, worksheets and calculations to conform with the extenders in the bill. It will be several weeks before anything is finalized at the IRS and with the TurboTax software which must also be approved by the IRS after they have completed their own system configurations.
Go to this IRS website for information - https://www.irs.gov/newsroom/irs-statement-on-retroactive-extender-provisions
See this TurboTax support FAQ for information - https://ttlc.intuit.com/questions/4241639-tax-relief-the-bipartisan-budget-act-of-2018