When you convert a traditional IRA to a Roth, a 5-year waiting period applies before you can withdraw any of your funds without a penalty. If you withdraw the funds early, you may be subject to the 10% early withdrawal penalty.
If you reach age 59 and ½ before you make the withdrawal, no penalty applies. After the 5 year period has passed, the converted funds can be withdrawn without penalty as long and the distribution is simply a return of your cost basis.