it's not very long at all, only like two years at the most. there's almost definitely a timeline somewhere. westeros.org and the accompanying forums would be a good place to look.
funny you should say that because between ASOS and AFFC grrm did plan on a time lapse - though it was more like 5 years than 20. he eventually abandoned that idea which is why it took so long between those books. but i mean, it's been just as long (longer now, i believe) between affc and adwd so.