Not sure what you're asking here. There are two 5 year rules for Roth IRAs.
Account must be open for at least 5 years for qualified withdrawals - your Roth 403B does not count for that purpose. The date you first opened any Roth IRA is what they're looking at here.
5 year rule on conversions to avoid penalty - does not apply here since a Roth 403B to Roth IRA rollover is not a conversion.
When you rollover the 403B to the Roth IRA, the basis will follow for the purposes of "regular contributions can be withdrawn at any time, tax and penalty free". I believe that figure should be on a form associated with the rollover, but as a back up, I hope you have good records of your contributions over the years. Maybe pull as many statements as you can find now and start keeping track on a spreadsheet.