The first amplifying vacuum tube was patented in 1908 and you could produce a crude computer from that, even if terribly inefficient.

I imagine civilisation would wind back to around the 18th century, because we would understand things like fertilizers, calculus and how to make steel, but could not immediately use that knowledge to produce steam trains, bicycles or saxophones.

So, assuming an accelerated pace of development thanks to some of this knowledge surviving I'd say 50 years give or take.