Merlin,
I guess it depends what size company your working for and how many employees they are worried about losing out to. However, a few years back I was working for a smallish on-shore operator and was bonded pro-rata for 2 years for types and an IR. Both parties were very happy with this.
I do get the impression that 3 seems nearer the norm on a pro rata basis, but trends can change quickly in this industry.
Rushes