On a goroutine not locked to an OS thread (the default), don't take more than 1 microsecond in a single C call. If you need to take longer in C, lock the goroutine to an OS thread (runtime.LockOSThread), but then don't do things in Go that would park that goroutine (time.Sleep, blocking channel read/write, etc).
If your C doesn't fight the scheduler it isn't that bad.