Getting a short string from a long one without spaces

I have a file with a huge string without spaces in it (something like: "A":123,"B":456,"C":789…)

I want to get X amount of chars before and after a specific value (like what is around "B").

But, if I use cat, because the string doesn’t have spaces, the whole string returns.

I tried cat time_series.json | grep "B" | cut -c1-50 but again the whole returned.

I tried cat time_series.json | grep "B" | cut -b 1-400 but it only returned the start (and my string is in the middle)

What can I do?