That'll depend on your Ordinary Account Balance. Let's suppose you're celebrating your 55th birthday later this year (2024). The Full Retirement Sum this year is $205,800. And let's suppose you "shield" all but $40,000 of your Special Account. (You cannot shield $40,000, sometimes more.) If your OA balance is at least $165,800 then your new Retirement Account will be funded to the Full Retirement Sum with $40,000 from OA and $165,800 from OA. If your OA has less than $165,800 then every dollar will be swept from your OA into your new RA. (Unless you tell the CPF Board otherwise, in advance, because you have housing payments you want to make from OA.)
If your MediSave Account has reached the Basic Healthcare Sum then the MediSave portion of compulsory contributions (and "all three account" Voluntary Contributions, a.k.a. VC3A) will land in your RA. If you're interested in injecting as much cash as possible into your RA then you'll need to raise it quickly at least to the Full Retirement Sum in order to avoid those payroll cycle contributions.
Note that certain qualified family members (especially a spouse) may be eligible to transfer his/her OA dollars into your RA. That's often a really great choice instead of cash, or in addition to a lower amount of cash. And it can be up to the ERS if he/she wishes. Just be aware that it may take a few days for the CPF Board to verify the family relationship if this is the first time that person is transferring OA dollars to you, and the 27th is very close to the end of the calendar month. You could try contacting the CPF Board to see if they'll verify the relationship in advance. Once the relationship is verified (and recorded in CPF's database) those cross-spousal (or other family-based) OA to RA transfers can be instant, or near-instant.