There is no need to wait for Form 5498 if you know the amount rolled over. Enter the Form 1099-R as received, indicate that you moved the money to another retirement account, then indicate the amount rolled over. TurboTax will include the entire gross distribution on Form 1040 line 4a (if the distribution was from an IRA) or line 4c (if the distribution was from a qualified retirement plan), but include on line 4b or 4d, respectively, only the portion not rolled over. TurboTax will also include the word ROLLOVER next to the line.
Of course if you are unsure of the amount rolled over, you could request a filing extension by July 15 and wait for the Form 5498 to be sure you are entering the correct amount as the amount rolled over.
