Exactly how large the permissible delay is will more or less be guided by the type of solution you are using.
- Updates must be (side by side) in real time : you need to either (a) have something that pushes data in MSSQL, but also click in MySQL, or (b) use replication to do this
- Updates may be delayed for a while : instead of trying to replicate things, you can turn them into an ETL process where you have several options, the best of which probably uses SQL Server Services Integration (SSIS), where you can create ETL packages to make the push any way you want.