I had an interesting experience auditioning for a band and they wanted me to learn new songs on the spot. That's fine but I didn't write things down and ended up forgetting parts so I didn't get the gig. Afterwards I thought I should've been given the songs ahead of time or some kind of chart to play from. They weren't the right people for me to play with anyways but there are situations where you can learn quickly. As long as you know your chords and have something written down you can play along to a new song pretty fast. Riffs take a little longer but in general, the more you're practicing, the easier it is to pick up something new.