This is a quick ab routine I like to do:
20 sit-ups
20 crunches
20 side-to-side heel touches (2-count: touching each heel once counts as 1)
20 flutter kicks (4-count)
20 Hello Darlings (4-count)
20 Mad Russians (2-count)
20 side crunches left
20 side crunches right
30 sec plank
30 sec side plank left
30 sec side plank right
Run through them start to finish with no rest besides transition time. You will feel the burn. I usually do this at the end of my workout. Right now I workout 5 days a week, so I hit abs 5x week. I usually only do one set, but every now and then I will do two. I'm also a fan of hitting a set here and there of various weighted ab exercises between my main lifts.
Your core (abs, obliques, lower back) plays a big role in overall strength and stability. Pretty much everything you do in regards to moving, you engage your core. If you have a weak core, you have a weak body and vice versa. IMO, you should train the core as aften as your body can handle.