Rough scales:
- Same data centre: around 1 ms.
- Same city: a few ms.
- London to Frankfurt or Amsterdam: around 10 ms.
- London to New York: around 70 ms.
- Home internet: varies widely.
How to check where the server is: ask support, or use traceroute from a VPS to the server address and look at the last hops. Ping from a VPS in different locations also shows which is closest.
A worked example of what to measure: compare slippage per trade from two locations. If the difference is negligible, the cheaper option is fine.
Do you know where your broker's server is?