Imam Ahmad (r) was asked about keeping the hair long for men. He said:
❝It is a good recommended Sunnah; had we (I) been able to act upon it we would have, but it requires effort (in looking after it well).❞
[Masa-il ‘Abdillah b. Ahmad]
So the Imam (r) considered it a recommended practice. But he saw that in his case, other recommended acts take priority. This shows that for those men who can look after their hair well, it is recommended for them to grow it long.