Though, ironically, msec is still ambiguous because that could be milli or micro. It's often milli so I wouldn't fault it, but we use micros just enough at my workplace where the distinction matters. I would usually do timeout_micros or timeout_millis.
We use "ms" because it's the standard SI symbol. Microseconds would be "us" to avoid the µ.
In fact, our French keyboards do have a "µ" key (as far as I remember, it was done so as to be able to easily write all SI prefixes) but using non-ASCII symbols is always a bit risky.
Shouldn't that be named "usec"? But then again, I can absolutely see someone typing msec to represent microseconds.
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can also do usec for micro