The question is a bit vague, however it depends a lot on what you are going for.
In terms of training for orienteering "Core" is a more appropriate thing to train than particularly abs. But we could go even further and ask why core even helps orienteering, a sport that mainly involves running and our legs.
For that I think there is a lot of info out there already that does a good job explaining what core strength actually does.http://www.smamiddennederland.nl/wp-content/upload...
Particularly... [core strength] helps to
stabilise the spine, pelvis, and kinetic chain
during functional movements
This particular stabilization is especially important in Orienteering which includes much more variation in footing and conditions.
(Some of the following is paraphrased from the book Running Rewired
if you are interested in checking it out)
I think what's important to note is that a lot of abdominal targeted exercises target very specific muscles that while technically part of our "core", these are not the muscles most responsible for stability and driving motion while running/orienteering.
What is actually doing a lot of the heavy lifting and is most often undertrained are the core muscles in your lower back, hips and glutes (erector spinae, psoas major, tensor fasciae etc.) but targeting these muscles individually also tends to ineffective.
The recommendation for at home, no equipment core work would include as many compound, stability, and rotational movements as possible (plank/plank variations, bridges, Russian twists, hollow hold, supermans etc) and less isolated movements (crunches). This is provide for a more effective workout that provides more strength with less DOMS (delayed onset muscles soreness) and an easier time recovering.
Anyways this brings me back to the original question and the answer is: it depends. Do core / abs as often as feels comfortable and aiming for a specific optimal volume of strength is less important than doing more effective strength training during that time.