how long do your convos normally last before you ask for the number?

i feel like there is never a natural moment for me. also, after you open with somethin glike id kick myself if i didnt come say hi, what do you nromally turn the convo too

I don't think it matters much. I don't think a robust transition to #close will fail you

"Ok, i gotta go back to [home/friends/work] now but we can have a coffee this week, what's your number?"
Or, a softer one:
"Ok, i gotta go back to [home/friends/work] now but we can get together later this week. Do you want me to call you?"

I completely disagree with asking for numbers, especially phrased like the part I put in bold. Make a statement, tell her to give you her number for a specific reason such as.
"Yeah (place you were talking about) is awesome, gimme your number and I'll let you know next time I'm going"

As for when to ask, I usually do it right before leaving, but some people think asking for it and then sticking around for a little while is a better idea.
After opening, you should be dropping some subtle DHVs while having her qualify herself. Ask her about herself, what she has going for her besides looks, what she likes to do, etc. Then tease some responses and flirt.

You may be right, but this is slightly different from asking the number.
From "Lovesystems Routines Manual"

One good way to get her number is to say:
“Do you want me to call you?”
This frames her response such that she has to essentially say
“Yes, I want you to call me.” This is a more powerful position
for you than simply asking for her number and waiting while
she either grants or refuses your request. This is one of those
elements of Love Systems which is very subtle but also very
important.