There’s been a lot of mentions in the news about Katla always erupting when Eyjafjallajökull does, but no time frames have been mentioned. It was a bit challenging to dig it up, but here it is:
Eyjafjallajökull’s most recent eruption, December 1821 to January 1823, was followed by an eruption of Katla in June and July 1823
Eyjafjallajökull and Katla: restless neighbours « The Volcanism Blog.
The one before that was in 1600s and it’s a bit uncertain if both even erupted then, let alone knowing which one went first.
With these 1.5 data points it seems we might very well have some time to fly around right now.
The eruption of Katla is well past the schedule, however, so if not right now (meaning in year or two), it will happen soonish, unless something extraordinary happens. There’s record of Katla erupting from 900s to present and it has been erupting twice per century all these centuries, and now it’s already 92 years from the previous one (1918).