alternatehistory.com

What is the longest period of time that two powers or power-blocs could fight over the same area, sometimes successfully conquering it and other times losing it?

The closest OTL examples may be the Chinese wars on the Mongolian Plateau, the Roman-Persian Wars, the Reconquista, and the Adal Sultanate and other Muslim kingdoms' conquests in Ethiopia.

As a rough ATL, perhaps the Reconquista is reversed and extended by having the Marinids defeat the Christian alliance in 1340, restoring al-Andalus. Continuous wars occur in southern Iberia until the Reformation, when weakening and infighting among the Catholic powers allows al-Andalus to expand to almost full extent of the 8th century. Yet, the economic growth during the Northern Renaissance, leads to the English raiding al-Andalus similarly to OTL Portuguese colonialism across the world, weakening them until French-supported Spanish states again retake the peninsula. The Ottoman Empire then reaches its peak and sends an expeditionary force to aid the ailing al-Andalus to retake Iberia. This continues indefinitely.
Top