Across the industry, most Pay As You Go networks will disable your account if you go 6 months without making a chargeable activity (chargeable activities include an outgoing call or an outgoing text message). On some networks, the inactivity period is substantially shorter: as short as 60 days. You’ll normally lose your phone number when your account is terminated.
The period after which unused credit will expire depends very much on the mobile network you use. On most networks, credit expires after 6 months of inactivity. However, it can be as short as 3 months or as long as 360 days. You can sometimes recover credit even after your account has been disabled: you’ll have a new phone number upon re-opening your account.
The following table summarises the inactivity period for Pay As You Go mobile networks here in the UK. Click on the name of your mobile network for further information on their inactivity policy. We’ve also provided a source link to the relevant terms and conditions: see further down in this article.

PAYG Mobile BB Dongle - Not used for a long time - Now not working
from Vodafone site...
"If you have no credit left and have not used it for 180 days then your account will be suspended but can be re-activated by topping up your account. After 270 days without use, and if you have no credit, your account will expire."
If none of the staff reply on here, I would call 191 on a vodafone phone and ask them if they activate it. You'll need the "phone number" of the SIM in the dongle.
You say there's 3/4 of the allowance left on there - does that mean it's a non-expiring 1GB dongle? The 3GB/30 day dongles would be void of any credit or allowance by now.
A Tesco Mobile spokesperson said “If a mobile number is not used, ie calls or texts made or received, within 12 months then the SIM is disconnected. If a customer contacts us to advise they wish to keep the mobile number, we will re-set up the accounts so long as that number has not been reassigned.”
With an 02 pay- as- you- go phone if you do not make or receive a chargeable call in any six month period it will also be disconnected and any credit lost. The number could be reallocated from then on.
Asda said “So that Asda know that the customer’s number is still in use, we just ask that they make either one call, which can be to the network operator at no cost to the customer one text or one instance of data usage every 270 days.”
If you're topping up using a debit or credit card, to help prevent fraud we have a daily top up limit of £30, weekly £60 and monthly £120 Any credit on your phone will not expire for 36 months providing your phone is used within this period.
I have an an old PAYG phone that's had a Virgin SIM in it for 3 years, I topped it up with £20 when I got it & it still works OK [It has £18.60 left]
1.....I'd be a bit careful - I have a PAYG with Orange and it got cut-off last year causing quite a lot of hassle. In my T&C it says it gets cut-off after 6 months, and I had definately used it 2 months previously. Orange tried to argue with me that I hadn't and that they had already given the number to someone else. I had to stand in the shop for 4 hours but it did get sorted out eventually... I send a text every 2 months or so just to keep it "active".
2.....I had an Orange PAYG and just used it when I didn't want to take my good phone. Tried to use it a week ago and it is cut off, contacted Orange and they said they get cut off if not used for 6 weeks.