Some writers swear by writing every day. The venerable Stephen King, for example.Other writers, countless others, worry about how long it's been since they last wrote a word. I think, before considering either of these cases, we need to look at what exactly writing is, and what our writing processes are like.
Writing is, at it's most obvious, stringing words together, and recording them somehow - pen and paper; keyboard and hard drive, etc. But I don't think that's all writing is.
Some people are true pantsers. They have an idea and sit down and start telling that story, just like that. For the majority of us, that's not the case. Many writers are plotters. They produce plot arcs and character arcs before they start to write the story. They write down all the characters' motivations and conflicts and setbacks. They know the high point of the action, and all the complications that will pop up.
Some of us lean towards pantsing, but we still have to do a bit of development before the words start to flow. Maybe a character sheet, a general idea of the plot. Me, I'm most comfortable thinking it through. I get to know my characters in my head, and have a vague idea of the story line. I think a story through for months, or even years. Sometimes I write other stories while I'm thinking. But even if I'm not physically getting words out on paper, planning is writing, whatever route you take.
If your book stars a chef at an Italian restaurant, and you go out once a week, and try a different dish, that's research. If you take a class in Italian cooking, that's research. If you read Italian cookbooks or look up recipes online, that's research.
Reading books by others in and outside of your genre, can also serve to get the creative juices flowing, as well as keep a writer au fait with what's out there. It's all research.
After you've planned what you're doing to some extent, you'll probably write the book. To the non-industry ear, the word 'write' sounds like it's all about adding new words. But according to E.B. White, "The best writing is rewriting." The editing stage also counts, despite the fact that it may not push your word tally forward.
Once again, process comes into play. Most authors let a book rest a bit, so they come at it with fresh eyes. After that, the paths diverge again. Some authors, sit down with a printed copy of the manuscript, and use a complicated set of hieroglyphics to show where the plot drags and what should be switched around. Some did a lot of mini-edits while they wrote, so at this point they check for consistency and dive into smaller edits. So far, my editing process seems to consist of re-reading, and then doing nothing while the book re-arranges itself in my head. (NB, I still have not completed an edit. )
It's all writing
I personally, can't write every day, in any form. Heck the only thing I'm sure I've done every day of my life, or even of the last year, is breathe. I know that we can be hard on ourselves when we haven't written in a while. But, if thinking it through and character charts and re-reading all count as writing, then you're probably not doing too bad.
Don't stress yourself if you find that you haven't put words on paper for a whole week, or month, or a few months. Writing is comprised of many different stages. If one isn't working for you write now, dive into another. Having trouble drafting? Then daydream. Not feeling very creative? Edit. None of the productive side going your way? Read. The only thing you need to be careful of is not to get stuck in the same stage, or avoid any one stage. (Having four complete 1st drafts and no complete 2nd drafts, I know a thing or two about this.)
If you can, write - in any form- every day. If you can't, just don't stay away too long.
When's the last time you wrote?