How long is a good amount of time until selling?

Many will pitch an offer right after a user subscribes. Often it is for a tripwire, which is an irresistible offer that is very low priced ($7 is a very popular price) and basically an impulse buy. However, it needs to be of very high quality, because if it is crap, then the buyer will unlikely buy from you again.

The reason for a tripwire is not make make money, but to turn a prospect into a buyer. This is because a buyer is something like 10 times more likely to buy from you again, than someone that has yet to buy from you. Additionally, the reason why you offer it right after they subscribe, much like after someone makes a purchase, it increases their endorphin and dopamine levels and are more likely to buy at that moment, than when they don't have their level raised. Why upsells are so popular. Hence after buying a tripwire, you should then hit them with your main offer. If they buy that, then attempt an upsell, etc...

You should let the subscription mature so that the subscriber develops trust on your brand/website. Bombarding the subscriber right away may be counter productive.

Do whatever feels comfortable for you. There is no right answer, but the most common option is to offer something free just after someone signs up. I always use a "PS... " that pitches something else anyway.

Think about what you would like to receive when you sign up to a list. If you don't like being sold to straight away, then don't do it to your own list!

At least wait a week before you start sending promotional emails to them.
The emails that you send to them during this week can make a difference on whether or not the subscriber will buy from you. Or even whether they will stay subscribed or not!
Have you built enough KLT (Know Like and Trust) with your new subscriber before you start making a pitch? Do they know you enough to trust your recommendations?
It's really about how you nurture your "list". Sure not all will read your emails but those that do will know (through your mails) whether you have their best interests at heart or whether you just want to hard sell to them to make $$.