Don't know if there's a standard, but I'd be interested to hear if there is.
My stance is this: it depends on the clients. Most of my business is with repeat clients with whom I have a good working relationship. They are also (usually) architects, and so the pay structure for design/consulting is typically a pay-when-paid deal. So they often don't have the cash to put up a retainer anyway. So I don't ask for one. We determine a billing schedule - percentage or time based, project milestone or monthly, etc. - and I bill according to that. They, in turn, bill their client, and eventually the money trickles back to me.
If I'm working with clients I've had bad experiences with for payment (I must be hurting for work in this case), I'll either charge a retainer (preferably equal to the anticipated cost for the first billing cycle) or require payment up front. If they're late on a payment, work stops until they catch up.