Got the van hire number plate and paid in advance, doesn't say anywhere on the receipt how long the Dartford crossing is valid for and not even on their website!
I think it was 12 months when I was using the crossing last year?
https://www.dartford-crossing-charge.service.gov.uk/PayforCrossing/PayforCrossing
For one off payments, any payment is valid for 12 months, whether its £2.50 or more, the system just deducts each crossing each time.
It is on the website, just worded a bit funny, its the first paragrath on the above page, where you initially pay
Theres also a bit called "check for paid crossings" where the next day you can check the system has taken the money, it doesnt seem to show up instantly, but when it does it will show the exact date and time and whether northbound or southbound, and your one of payment will be reduced.
For peace of mind its worth checking on line before midnight the next day, especialy if its a hire van as the Pcn will go to them and finally to you with an admin fine on top.
If it dont show at least you have time to take evasive action, though you have a receipt as proof of payment.
For future, if you think your gonna do more than about 3 return trips and can afford to leave in about £10 its worth opening a free account, you can add yours or other relatives vehicles permanently ( gatwick airport trips etc) or any vehicle really providing its done PRIOR to crossing, you get about a 1/3 off about 80p each way in a car or van