In ENT episode “Divergence” parts go flying off the ship during warp 5. What would happen to something at that speed without the protection of the field? Would it stay in high speed motion until interacted upon by something else (meteoroids/dust/gravitational fields)? Or would it disintegrate under this kind of speed?
I think we’ve seen enough to safely say it would fall out of FTL in fairly short order.
I don’t think we’ve seen enough to say exactly what its sublight velocity would be - no examples are coming to mind.
Edit: as is often the case when the warp drive comes up, its worth pointing out that ships at warp pretty definitely have momentum/inertia.
It’s arguable that it would stop completely, if it survived falling out of the warp field.
The times we’ve seen ships fall out of warp for one reason or another, they typically slow to a stop, rather than keeping all the inertia like we would expect them to, or stopping instantly like they would if the warp field was doing all the work.