From 30 to 150 meters, but 60 meters or less seems to be often preffered in big cities
The citadel of medieval city of Carcasone has wall in length of 3km with 52 towers, which would give you the average distance of around 60 meters, during the Roman times the spacing between the towers of castellum were between 18 to 30 meters.
14th century fortification of Vyšehrad in Prague had 15 square towers spaced 60 meters apart plus two fortified gatehouses.
Wall of Philip II Augustus in Paris built between 1190 and 1213 had 77 circular towers in 60 meters intervals. Wall was considerd adequate long into 15th century.
Forification of Brussels in 14th century was 4km long and had 40 towers and 7 fortified gatehouses, that gives an average of 85 meters.
Of the walls of Tallinn built in 12th century there is 1850 meters still standing today with 26 towers, that is average span of some 70 meters.
Based on tourist information a wall in Rothenburg has 4 kilometers and 42 towers, which would give an average interval somewhere under 100 meters.
City of Krakau was fortified in 13th century by 2km of wall with 50 towers and 8 gatehouses - average of 40 meters.
City of Citadella has its walls built in 13th century in total lenght of 1461 meters and has 32 towers evenly spaced at around 45 meters.
Based on those examples I dare to say that in medieval Europe, if city was sufficiently wealthy it aimed to one tower at least every 60 meters.
You can browse the List of walled cities for more inspiration.
(beware not all listed has medieval fortifications)