I would say from experiance turbocharging lots of standard engines (including silver top zetecs and variations based on that engine) that it is very much down to the tune as to how much boost you can run. Simply put you can destroy an engine on low boost if the tune is wrong. But pysically the strength of the pistons is the limiting factor, the ring land breaks on them with too much boost (ive had this) this occured with me at around 17 psi, on a 2 litre, but ive ran a 1.8 at almost as much boost with no problems.
To sum it up i would suggest 12 psi with a good tune will last indefinatly, but upto 15 psi can be ran with a good tune if you have a fair bit of mechanical sympathy and are not expecting to drive to the moon and back. Any more boost than this will last, but for how long depends on many factors, driving style, tune, intercooling and many more things. I would stronly suggest aftermarket managment with any turbo conversion of this nature.