How much time, on average, should a moderator devote to their site (especially one that is new)?
This depends entirely on you, your personality, the focus of your site, how quickly your community is growing and how often "moderation" needs to be done.
What contributing factors should one take into consideration?
Moderation factors include aspects you mentioned - flags being raised, meta type questions that need to be answered, content curation duties. But they also include intangibles such as what the community's expectations for response times, are there others that share moderation duties with you, real life responsibilities, and your disposition to doing something now versus later.
Moderation time boils down to work/life balance. This is especially true if moderation is volunteer work and not part of your livelihood (eg. a job or new business venture). If you are volunteering to moderator a community, I don't see a reason to ignore your life. Set community expectation that flags may not be monitored and handled 24 hours a day. If you are moderating for a job or business venture, it's harder to take a break, but some balance is still required. Are you being paid to monitor the community 24 hours a day, 7 days a week? If not, why are you?
Setting community expectations about how quickly you handle flags can be done very simply. If you handle things immediately, they will expect immediate responses. If you let less serious things dangle while you gather information, compose a response, go for a walk, talk to your significant other, your community should respect that. They have lives outside of your community, you should as well.
If immediate responses are the expectation, it is time to consider more moderators (or automation). More eyes to deal with the problem, more quickly, will help restore your balance. You'll be able to spend less time doing the moderation dance because you'll have a team (or system) that can help you.
The passion of building a new site and community around it is important to maintain. However, this passion can be redirected from "moderation" to community building. Engage your active community members and see if you can come up with ways to expand.
Another aspect to consider the size of your community. If you have a small community, you need to ask if you are moderating the right things if your time is being consumed by their flags/questions/actions.