Phone roaming question

[allnewname]

I'm away for 2 weeks and my provider (3) has a 2-week add-on which will allow me to use my data while I'm away.
My question is... I'd prefer to get it purchased and set up before I go, but will it activate straight away (this wasting a day when I can't use it anyway as I'll be in the air)? Or will it only activate when I turn roaming on when I get there... ?
Does anyone know?
I know I could use their chat function but hoping someone will know here. TIA

Sorry not 100% but my EE one is a month and starts right away. Maybe you could do it from the airport before getting on the flight to be safe? Also first time I used mine i forgot to cancel so paid an extra month 😡 not made that mistake again!
Did Find this though on their website:

Hadn’t let image upload 🤣

Ah thank you so much.