Game designers have had a long tradition of setting the length of a combat round based on "feel". Rounds range from 1 minute down to 1 second in length. But how long should a round be, in order to make combat have verisimilitude?
In a professional boxing match, a "round" is three minutes long. During an average round, around 70-80 blows are exchanged. That comes to about 1 blow per 2.4 seconds, or roughly 5 seconds per exchange of blows, on average. If you intend to make fighting specialists get more than one attack per non-specialist's attack, perhaps doubling that length would seem appropriate. Or, perhaps, lengthening the round length a little might work, say 6 seconds. If a round might be taken up by non-attack actions, then shortening it would be a good idea, relative to how many rounds would not include an attack.