First time she takes you home to meet her parents, time it just after the main course of the Sunday roast, with a slight lean to the right to allow for control, tone, and pitch. Failing this the first time she puts her head under the duvet.
It doesn't matter when you guff in front of her, all birds are secretly aroused by breaking wind..... You should however prepare yourself for the first time you follow her into the bathroom or the first time you watch her face contort as Bachs best erupts from her delicate trump tube.
You could start a diary for the future kids / grandkids with relationship firsts... ie.
1. First time you nailed / plated her on the blob
2. First time you licked her apple and poked her farthole
3. First time you treated her to a dutch over
4. First time you tried on her undies.
5. Try and capture the look of horror on yoru face the first time you leave a skid mark on the bedding when she's been riding you for twenty minutes.
She will keep a similar book about first time you told her you loved her and a petal from the first flowers you sent but I gaurantee you book will be the better read
The best time my friend is when you are confident of making an impression. Do a girlie fart and she'll dump you, wait until your guts are full of putrified gurgling gas and let rip one of those where you lie back and think "fcuk me is that the smelliest one ever done or what".
If she loves you, she'll high-5 with you every time your bottom burps.
Make sure you're on top though. A golden guff should not be spoiled by her running out of the room gagging. So be on top so you can pin her down while introducingnher to the waft of your rear innerds. She will thank you for it in the long run. And if you shoot your load at the exact same moment you can mix romance too.