Short answer: as always when playing music, it takes years to perfect. But since you're doing this for friends and family: take your guitar, sing and enjoy (probably while making mistakes, don't worry). There's no way to push yourself to become perfect in two weeks, but have fun and take time to perfect yourself in the future. I 've a suggestion: whatever the outcome is, promise to repeat this next year (they'll be amazed about the progress you made!). I've been singing and playing for 30 years now; although it sounds pretty good (I suppose), I'm still a better singer without my guitar and better player when I don't sing...that's normal.