Lag is always present. Every trade/quote has a timestamp associated with it. If you compare that with when you actually receive it, that will give you the lag (make sure you have a good NTP time source to sync to, of course).
Firstly, there is the lag from an event (trade/quote) being initiated and when it is reported by the exchange's equipment (SIAC/SIP etc.).
From there, there's the actual transmission delay to the first consumer of the data.
If you have hardware co-located at an exchange's data center, it's measured in nanoseconds, plus processing time on your hardware (and software, if applicable).
If you use a data vendor, they're the ones with hardware typically located in an exchange which takes the raw data and sends it to other systems at the data vendor for dissemination to clients.
For institutional-level realtime feeds, you can expect a handful of milliseconds.
As you move down the chain to retail level realtime feeds, you should expect a few hundred milliseconds.
Finally, there's the connectivity lag (Internet or dedicated link) from the data vendor to your system and processing time on your system.
In short, you should ask your data vendor - they should be answer a simple question like this, and give you expectations of the lag involved, especially at key times like market open and close.