Ahead of time and during initial interviews they rarely look at them has been my experience.
It's more likely that they'll look after the interview, once they've met you, seen if there's a personality and basic skills match. Looking at projects and code usually comes after this. Frequently they will expect you to walk them through any given project and describe your specific duties and tasks in a 2-way conversation fashion.
Other factors that will influence this process are:
Number of candidates:
If there's 20 candidates, they usually will not spend the 30-60 minutes reviewing each ones portfolio. Again, first they'll try and winnow down the candidates that can meet the basics. If there's just 2, there's a higher chance they'll be looking at your work ahead of an interview.
Type of job:
Backend end processing type jobs are less likely to look at front end work, whereas a web design developer would be expected to have an extensive and impressive online library and examples of their work and personal design projects.
Seniority of the position:
They're way more likely to do this review if you're applying for a principal/chief/etc role than if you're applying for a junior position. That's because people at the higher levels will influence more than just their assigned projects, so you want to get a fuller picture.
If it's the first round and every candidate gets 1/2 hour, the interviewer is unlikely to look at stuff ahead of time. However if it was a 3rd round 'final' interview before offering you a position / determining your salary, the interviewer is much more likely to spend some prep time researching your stuff.