Otherwise it's a pair-programming test with massive hand-holding from another developer
It that's all it is, then what is the benefit of doing it for more than maybe an hour or two? Are you really going to learn much more about the applicant's general level of skill and experience, or change your opinions on whether they'd be good to work with, if they are with you for 3 or 4 or 8 hours instead?
On a separate point, you are making an assumption that in a pair programming situation the hand-holding will all be one-way because the applicant doesn't know the code base. Hopefully anyone you're thinking of hiring would also be able to show the partner/interviewer a useful thing or two in a whole day of coding based only on general programming principles.
Also, if the applicant has a general knowledge of what the business does (which hopefully they bothered to research before applying!) and the interviewer and code base are any good at all, the applicant will understand enough about the problem domain to being to work "natively" within a day. Of course they won't suddenly understand everything, but interviews are two-way deals, and if someone the employer puts forward to represent them can't explain their own product well enough for native thinking to at least begin, or if the code base is so bad that a decent candidate can't start to find their way around after a whole day, then the employer failed the interview.
It that's all it is, then what is the benefit of doing it for more than maybe an hour or two? Are you really going to learn much more about the applicant's general level of skill and experience, or change your opinions on whether they'd be good to work with, if they are with you for 3 or 4 or 8 hours instead?
On a separate point, you are making an assumption that in a pair programming situation the hand-holding will all be one-way because the applicant doesn't know the code base. Hopefully anyone you're thinking of hiring would also be able to show the partner/interviewer a useful thing or two in a whole day of coding based only on general programming principles.
Also, if the applicant has a general knowledge of what the business does (which hopefully they bothered to research before applying!) and the interviewer and code base are any good at all, the applicant will understand enough about the problem domain to being to work "natively" within a day. Of course they won't suddenly understand everything, but interviews are two-way deals, and if someone the employer puts forward to represent them can't explain their own product well enough for native thinking to at least begin, or if the code base is so bad that a decent candidate can't start to find their way around after a whole day, then the employer failed the interview.