It used to be that there was no time limit, the first response was initial but connection was done on a first come first serve basis. I.e. if you'd not confirmed and paid your money and a couple of big installs went up in the area then when you try to confirm the DNO could turn round and say sorry things have changed and we can't accept the installation. I think once you've paid the fee as far as they are concerned the installation is done.
This was the feed back i got from them about 18months ago. I don't administer it anymore so not sure if it has changed or not. Basically i don;t think there is a time limit as it is possible it could change at anytime.
If you need a SMA 10000 Central i've got a spare one kicking around. Thought i'd got rid of it, but the client's accountant from some reason advised him not to go with solar, cause he would have been penalised by drawing down his stocks on a low stock market. Crazy! who says it's not going down further. South facing, south coast, steel roof, DNO allowed 20KWp into the single phase. Massive daytime electricity bill... First year returns of over 14%... Sometimes accountants are so stupid...