So now I think I understand...

[Niall]

There is no right time or wrong time. You just need to MTFU and do it. End of the day, what's the worst that can happen? She says no, you both just crack on as usual.
I did the same with my misses. Spent ages thinking shall I do it and in the end I left it too late. Thankfully 6 months down the line she changed her mind.
Give her a ring and see if she's up for a drink tonight, take her out, date ra*e her then ask! Even if she Questions you about it the next day, you can just deny it. Easy!