Thanks for all the responses so far, it's interesting to see so many different viewpoints.
I'm not sure that an advice charging structure would work in our specific situation. We've already started charging for some things that customers have traditionally expected for free and take items in for a chargeable repair instead of fixing it over the counter where possible. We also have a tutorial fee for teaching people about new equipment (TVs etc). Allowing people X amount of query minutes before we start the timer would just cause us to get a bad reputation, bad online reviews and encourage people to shop elsewhere, as others have said. The service aspect is our USP that the chain and online stores can't compete with, so I don't want to go too heavily down that route. I'll take another look at some of the services we offer though, I think there's room for improvement in some areas like key fob batteries etc.
I'm very confrontation-averse so I suppose I'm looking more for tips on how to subtly shut down an interaction when things start looking like turning too time-intensive, or when the customer is obviously trying to take advantage. Some people are really persistent and no amount of, 'I'm sorry, but I can't really advise without seeing it/can't draw you a diagram/can't consult my magic crystal ball' seems to work. I read a great advice thread a couple of years ago (not here) from someone asking how to politely deflect her boss's increasing demands, and there were some great body language techniques and phrases discussed that took control of the conversation, but were tactful enough to avoid offence. I wish I could find it again.
To the person asking if our customers expect us to change their bedpans for them ..yes, they would if they could! A lot of them are elderly and genuinely struggle with basic tasks, which is part of the reason I don't want to get too strict or rude with anyone. We also have a school for the blind nearby, so we get a lot of customers coming in with visual impairments. I try to be as compassionate as I can and help people out when they genuinely can't do something themselves but there does come a point where I feel like an unpaid social carer, which is not compatible with running a business.
It's also a very arty/hippy town, so we definitely have our...well, let's just call them 'characters'. The Chatters are common, people who talk so much and for so long that we genuinely don't know what on earth they're on about. It takes them ages to get to the point of what they actually want, and in the meantime they'll tell us all about their Uncle George's girlfriend's basket-weaving hobby or whatever, which they seem to think is related to their query (it never is). We're constantly trying to steer them back on track.
I have definitely used the techniques someone mentioned where another co-worker phones me or I just walk away into the office, but in many cases the Chatters still stand there continuing the monologue with themselves, or just raise their voices so I can hear. It's so annoying when the task I need to do is out on the shop floor and I have to pace the office until they eventually leave! I've also just ignored them and carried on doing what I need to do, but they follow me around the store, still talking about god-knows-what. I've now started walking towards the door while they're talking and if I open the door for them, like I'm trying to be helpful, they'll often wind up their own conversation and shunt themselves outside without even realising what I've just done. I laugh every time it happens.
As for interns, I'm in the UK so unpaid interns and school credits aren't really a thing. I'm looking to hire someone new at the moment but we have a fairly decent minimum wage so I won't want to pay that person to deal with the time-wasters. I like the suggestion of telling people I can look at something 'later' for them to put them off as I can perhaps make up an excuse that I have a time-sensitive order that needs to get submitted in the next 15 minutes or something so they don't get offended. That would work in some cases.