This is the rather famous 5 intersecting tetrahedrons. It's quite possibly the most beautiful model I've ever seen and I'm still amazed that I even managed to put it together. It's made of 30 units and took me a week of off and on work to fold all the units and especially put it together. basically, 2 days to fold the units and 1 day for each tetrahedron. I tend to split up larger models like this so that I don't get frustrated and end up ruining it.
The math behind this model is amazing, and the symmetry breathtaking. It looks better with 5 different colours, but I really like using the word-a-day calenders or what have you since they are cheap and have 365 identical squares. works well.
Credit where it's due, i used this tutorial for it: [link]
Made with 30 units