Totally depends on the job you are applying for.
I would aim for 2 pages, but most recruiters will be OK if it runs on to 3 pages (if it's relevant).
Key thing is not the page number but the content and the structure. Particularly if people are looking at lots of CVs, you need to show them in the first 10 seconds of reading, why you have the skills/attributes for the post they are filling. This hardly even gets them to the second page of your cv.
If you imagine a recruiter looking through a pile of cvs - they are going to put them in 3 categories: yes, no and possible. If they get enough cvs in the yes pile to make a longlist for filling the post they will only have a quick look through the possible pile, and only a cursory glance through the no pile. (There is some evidence that this is the case even when there is a 'rigorous' structured scoring system applied to cvs.
Typical recruiters will then have a longer look through the cvs in the yes pile (perhaps even a minute or two per cv!) to confirm their initial impression.
Are there ways in which you can condense parts of your cv to reduce space, but still retain meaning? e.g. "I have two post-graduate qualifications in marketing and engineering". or "I am a respected researcher and I am an author of numerous publications, including 3 books and 17 papers in peer reviewed journals".
As I said a the start, it totally depends on the job you are applying for. Gone are the days when you can write a single standard cv and send it out to all employers. Each cv needs to be tailored to the specific job you are applying for - highlighting certain parts of your experience, using language and terminology that matches what they are looking for etc. And yes, it does take a huge amount of time to do that. If you get an interview for a job you want it is worth it.