They have 6 months from the date of the offence to lay the information before the court. From a practical point of view that means that you can normally breathe a sigh of relief about 7 months after.
This all depends on your circumstances. For example, if you tripped a camera and then ignored all letters from the scamerati then they could summons you 6 months + 28 days from the date of the speeding offence, as that's when you would have failed to provide information. The charge would be failing to provide driver info as requested under S172. If you did, however, provide full details of the driver as requested, licence No, DOB etc then the information would have to be laid within 6 months of the date of the actual speeding offence.
So, to sum up, speeding you are free and clear after 6 months. S172 after 6 mths +28 days, give or take (come on, you didn't think it'd be that straightforward, did you?

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Clearer?
Please don't assume that, if you haven't heard from them by their deadline, that all is ok. It's common for a summons to turn up at '6 months + 3 weeks' but be dated '6 months minus 1 day' and so be within the information laying rules.
Not that I am suggesting an abuse of process by the cops/CPS etc. No sir.