How long does my data last on PAYG? + How long does my number last?

[starmaker]

£10 for 18GB is the latest offer.

The bit I'm confused about is 'for up to three months'.

If I ONLY topup £10, does that give me 18GB to use for 3 months?

ALSO: let's say I top up £10... and don't top up again and only use my phone for receiving calls, how long will my number last for without topping up?

Thanks.

[MI5 · Accepted solution]

@starmaker 

You get 18gb for 30 days. You need to top up every month to renew the allowances. For 3 months, you'll get 18gb each month when you top up.

Your SIM will remain active for 6 months if only used to receive incoming calls.