Cycling with atrial fibrillation (AFib), arrhythmia and other heart issues

I wonder, anyone know of good data of how the risks increase with number of hours of exercise?

Nuanced... (one of my pet interest areas-AFLETES published in May was most recent update I'd seen but below is a collection of other interesting ones):
TAUPO study:

There was no sign that admission rates were higher amongst the more active participants than those who cycled less. For every additional hour spent cycling per week, the risk reduced by 0.90 (95% confidence interval 0.79 – 1.01). After adjustment for age and height the hazard ratio remained 0.90 (0.79-1.03). The model was run separately to control for BMI, and again this did not change the association between time spent cycling and hospitalisation for AF. There was no evidence of modification by age (50 and over compared with under 50 years).

2000 hours:

It was also found that moderate to severe activity in excess of 2000 hours during lifetime was significantly associated with a higher incidence of AF (odds ratio=3.88; 95% confidence interval= 1.55-9.73)

U-shaped association and deep overview:
Graph actually from this article:

Also supported here:

and a Clinical trial of sorts- which concluded:

Despite changes in LA structure and mechanical function after only 10 months of exercise training, there were no clear changes in atrial electrophysiology; a longer duration or dose of training may be required to induce LA electrical changes thought to be causative of the increased risk of AF in endurance athletes.