You have to register either within 3 days of being put on the SOR or within 3 days of leaving prison, whichever applies or is the later date. For an indefinite notification period, that would be the start of the 15 years before the possibility of applying to come off the SOR.

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If you are to punish a man retributively you must injure him. If you are to reform him you must improve him. And men are not improved by injuries. (George Bernard Shaw)

Just reading this ... Not sure I'm answering the right question (!?), but my understanding is you have to register and then re-register 12 months later etc withing 3 days of being put on the SOR
Hope that helps

I did this once on a bank holiday (unintentionally, am self-employed online from home with international clients, so didn't even realise it was a bank holiday) and the desk guy was OK with it, as he said he was on double pay by the hour, so it went into the next hour and he would make another cup of tea and stretch it out.

Remember to arrive five minutes before closing time. Then take your time to slowly go through the paperwork. Weekends and BH are the best time as the staffing is minimal and your keeping then from going home greatly appreciated by those on duty. 

There's no limit, you just need to meet the notification requirements
There is a list of ' approved ' police stations though

I went to my local police station recently as a requirement. It is probably going to be the last visit as it will soon be closed. So my next visit although a while off is going to involve some travel to the next "nearest". I suppose the answer of how far are we expected to travel is "as far as it takes".
Does anyone know if there is a limit, I know this is a long shot (no pun intended)