Turn length also depends on what your goals are - if you're trying to go fast in a group of 4 or more, 20 or so revs per pull. Two riders, 30 seconds was the rule of thumb (that was a while ago). Keeps you at or close to your limit without pushing you over.

However, on long rides where speed isn't the ultimate goal, then longer pulls.

Also, if one is feeling stronger than the other, they should pull longer.

congrats on having a fun 75 mile ride!

hope this helps,
cdr