Well that depends on what I'm writing and how much is needed! I deal with research in a couple of ways. If I know which story I'm writing next, and I know there are a couple of areas I'll need to research, then I'll try to find a book that I can read while I'm finishing off my current project. That way by the time I start the new manuscript, I have some idea of what is involved. I did this with Into the Fire. I found the tribe I wanted Tai to be part of and read a book on their history to give me an overview of some of the things they had faced. I also found a book for The Flanagan Sisters which included the stories of women from each class in El Salvador.
Then there are the things that I need to research while I'm writing. In my current manuscript, my hero is an artist and my heroine is a software billionaire. I needed to know a little about an artist's process, galleries and exhibitions and software programming. As I was doing my dirty draft, I wrote what I thought was accurate. In the first draft stage I don't like to spend hours researching because I can get stuck down a rabbit hole and never get the book written. So I did my best guess on all of these things with a note to check the details later.
When I finished, I did my research. I have a friend who is a painter and so I chatted to him about his process, how long it takes him to complete a painting and the paints he uses. I then went to an art gallery, chatted to the woman there and asked her if she would mind answering some questions for me. She was more than happy to, so I emailed her through the list and she answered them when she had time. I'm still researching a few more details for the software billionaire, but a small part of the story involves indie game development and I watched a great documentary about it that really helped. In all cases I only needed to tweak what I had written a little, to make it right.
The one thing I always ask myself when it comes to research is, how much do I need to know to write an accurate story, and how much does my reader need to know? Generally only about ten percent of what I research ends up in the book and hopefully the reader doesn't know how much research I've done. They should be caught up in the story. I hate it when I'm reading a book and the author takes three paragraphs to tell me the history of something that has no impact on the story whatsoever, so I do my best not to do it myself.
When I'm editing the manuscript I'll sometimes stop and go online to find a few more details, or to double check what I've written is correct. In the dirty draft stage I will often write XXXX when I don't know something and don't want to interrupt my writing to check. I've written the XXXX when I can't come up with a name (particularly company names) or when my characters are eating food from a particular country and I can't think of anything off the top of my head. Then when I'm finished writing, I'll search for the XXXX and work out what to replace them with.
So that's a little about how and how much I research. I guess my biggest thing is I don't let a lack of knowledge stop me from finishing the book, but I always make sure I check those details before I send the manuscript to my publisher.