Sorry - update - rebooted a few times and also switched chars and seems to be back - sorted 
Hi there
I had a character boost to take me to 120 and in fairness has been there awhile as was not sure which char/when to boost - finally made a decision and the prompt is no longer showing against my list of characters.
Is there a deadline to use these as I didn’t realise 
… thought it would stay there till I made up my mind
Hi, Nilari! o/
Glad to see you’ve found your boost 
It can disappear sometimes when there’s some kind of interface issue, but logging in with a few characters or changing realms can help. Otherwise, you can always try a full interface reset.
In any case, just a clarification. Boosts don’t expire. I still have the one that came with Warlords of Draenor on my account. They will have to change once the level squish happens in Shadowlands, but we still don’t have the details about how that will work.
Many thanks Agaleith for the clarification 
Have a fun festive period