Where do you post from? I go to my local post office so I end up using Royal Mail. Their online price finder can give you the price for shipping and also an estimate. The estimate is never always going to be spot-on, it does depend on the individuals carrying the item, really. Some things get waylaid, others get lost, some get sped along at a mighty pace.
Most of my products go from the UK to America, and it usually takes about a week to arrive. Royal Mail estimate, with the option I use, about 5 working days. So far, judging by when feedback has been left, everything has arrived between a week and a week and a half. If people ask me how long it will take I'd just say two weeks. It's not long enough for them to get angry about (unless they wanted it by a certain date, in which case they should have ordered it sooner, or request faster delivery), but not so short that if it gets temporarily misplaced and then quickly found again that they'll get irate and consider you a bad seller.
Otherwise, going by experience from how long it's taken things to arrive the other way around is as good an idea as any, since you can never -guarantee- when it will arrive. I've had things sent "next day delivery" - I've been given a tracking number, which even says itself that it's next day delivery, and it has taken about 4 days to show up. You just have to hope that if the worst does happen that the customer will understand that it's not your fault and give you the chance to fix it instead of instantly jumping to negative feedback or requesting an instant refund.
