It depends on the airline.

If it's DL, you will generally not be offered a challenge unless you have elite status on another airline. You need to fly 6,000 MQMs for FO and 12,000 MQMs for GM within a 3-month period. Depending on fare class, you get 1 or 1.5 MQMs per flown mile. However, DL takes 3-4 weeks to process your request and will not make it retroactive, so if you're flying in 2 weeks you may be out of luck.

On AA, anyone can take the challenge and you can sign up immediately. It requires you to earn 5,000 q-points for GLD and 10,000 q-points for PLT within a 3-month period. Low fares earn only 0.5 q-points per flown mile (high fares earn 1.5).