Can I leave my book in my regular hot press for a “cold press” just for a longer time?

Yes! You can use your regular hot press as a cold press.

The one caveat is that you should lower the pressure significantly. The primary advantage of a cold press is freeing up your hot press to press more books. Also, they look kind of cool when cold pressing!

I recommend between 40-60lb in my hot press (depending on the book) but I recommend only about 2lb for a cold press for the “big wavies”. Higher pressures are not as effective at global page waviness as the page is under tension and it is shrunk slightly at high pressure. In order to return to its normal state it’s better for the book to be flattened but not under tension. Cold pressing for big wavies is about 3-5 days for me typically at ambient temperature and about two lb of pressure. If you are concerned about spine tic reversion then you will profit from higher pressure (something between 2-5lb of pressure over the area of a typical comic book) for a shorter time. Its a balance between those and the wavies at that point.