Call the Burbank office and talk to them. They are very friendly and will give you a straight up answer. It is all based on need, if they need a turbine pilot at the moment you call you have a better than average chance.
PDX is a good base to build multi-piston time, but the wait for a turbine transition runs longer than the average AMF base, plus the turbine runs are not good time builders (about 2hrs/day). Most piston pilot's at PDX tend to migrate to a regional, or seek a base bid at another base in the AMF system-this is the key to getting the turbine PIC on the quick.
From what I've seen, it seems like most of the 99 runs aren't "good" time builders in the sense that you typically get around 2 hours a day. Portland has a few that are 3 hours +, but most are between 1.8 and 2.2. Most bases seem to be like this for the 99. And no, they aren't looking for turbine pilots in Portland, just Chieftain.
Ameriflight is a good place to build great time. Low timers can go from a piston to a turboprop in a relative short time depending on company need. You will fly mostly nights and will sleep all day, so you won't really have a life throughout the week but you will be building that time, which is a good thing. Most of the people I have met have been great but there are a few exceptions...Be very leary of the San Juan Base. The base MGR (I use the term loosely) is the biggest A$$ in PR. He suffers from a "short man" complex and treats the pilots like SH@^!! Ask anyone that works in PR or anyone that's been here or knows of J.M. (the flight attendant). The opinion is always the same (so don't take my work for it). All in all, Ameriflight is a really good place to build time...