Ideally how long should you work in a charity shop?

How long you say will depend on how long it takes you to find a paying job. You may plan to get one in 3/4 months but end up having to wait considerably longer.

I didn’t mean i plan on finding a paying job within that time frame. I meant that’s how long i plan to volunteer for.

Well I mean you are putting in a lot of hours for free really. 10-5. Have you asked about your breaks/ lunch-break. They might work you hard too (aka kind of abusing it). But I mean that's 14 hours a week free. Which even at say £4 an hour is £56 a week you are contributing. That's assuming well below an adult minimum wage, but just giving the numbers for understanding.

When you note down (or atleast when I do and from what I've seen other people do) work experience/ voluntary work I don't note down what hours I worked/ how many hours I worked but rather just the duration/length of the work.
So I would say Jan 2018- March 2018 Volunteering at Cancer Research

If you're doing this out of the goodness of your heart, then, I guess the hours (opportunity cost) given don't matter too much.

Honestly though, I think you would be best seeing if you can cut down the hours. If you're young you kinda don't have a lot of money and giving away that much time for free (it's definitely not in holidays cos Jan?). How old are you?

I’m 17. I don’t really mind not getting paid as i am really only doing it for experience. I have had a trial there and there’s 2 other volunteers and 1 actual paid employee. During the trial i ended up staying for an extra 2 hours because it was hectic and they needed help as one of the volunteers was struggling alone. I didn’t get a break throughout the 4 hours i was there. Also when i was there the volunteers only got a break once during the day and that was it. I doubt they got a second break when i left as there was only an hour until it closed. Initially i did ask them if i could just do a morning shift on Tuesday and an afternoon shift on Friday but they told me i could do 10-5 on 2 days instead.

I start my first volunteer job tomorrow. 10-5 on Tuesday and Thursday at a charity shop. However, sometime this year i’d also like to get a paying job for the first time. How long do you think i should stay? I was thinking 3-4 months. I’d probably leave in March, April or June. Or is that too soon? I did ask family members and friends but they said that’s too long?

Okay. That's good.
Can I ask how you get 10-5pm on 2 week-days free? Like, are you on a gap year?
Yeah, I guess you appreciate then that is how the job is going to be pretty much- hectic and tiring! So it's up to you to figure how much of that you can handle with the reward being experience.
As an fyi, I got my first job without any work experience (over the summer of my 1st year of uni, so I was 19 - and still am, lol).
I feel like with a job like this the experience element is useful but you'll learn a lot experience diminishing returns pretty quick. So I would say about 3 months is an ideal amount as it shows you are reliable, can stick around, and have experience, which is probably what employers are looking for when you want a paid job in the future.
Personally I wish you goodluck as you are doing a good thing but it wasn't for me and it is not essential for getting a paid job as hopefully I've showcased a little. It will help in an interview though surely for a paid job as you can refer back to what you did