How much dating is long enough before marriage? My brother went 2 yrs. with his current wife. I'm 10yrs older than him and still single, but I have a better career. So how long?

I’ve been dating a girl for about 3 months.
I’ve been in 4 long term relationships for the past 8 years.
She has been in the same situation.
We are both Taurus.
She’s 5 years younger than me.
I’m 35.
She’s help me realize things about myself I kind always knew but didn’t put out there.
I’m helping her with her issues.
How long is long enough to date?