How frequently are airline pilots switching between aircraft types (e.g. Airbus, Boeing, etc)? Are pilots at Airlines typically assigned to fly a single Type in a given month / year or do they get switched routinely between multiple types of aircraft as per scheduling convenience?

i.e. Is it common to, say, fly a B-747 &amp; then return flying a B-777 etc. 

How about more extreme changes like switching between Boeings &amp; Airbuses or Turbojet / Turboprop etc.? 

I guess that begs the question whether pilots typically are certified &amp; current across multiple Types? How common is this?

I'm looking more for comments about what is operating practice across airlines rather than a hard rule, which I guess does not exist.
Airline pilots are typically only certified and current on one type. It is rare for a pilot to be certified and current on multiple types because they must do re-current training and check rides for each type in order to remain current and legal. The extra cost for training and check rides would not make financial sense to an airline trying to be competitive with other airlines.