I'm a long-time kevlar user who has experimented with poly many times.
My advice: don't bother with poly.
Poly string is the polar opposite to kevlar as far as the playable lifespan goes:
Kevlar has a 1-2 hour break-in period. After the break-in period, the tension is stable and reliable for many hours until it breaks (which can be a really long time if your use 16g).
Poly has a 1-2 hour period when, at the right tension, it plays well. After the first 1-2 hours, the playability gradually declines with time. How long you can go depends on your tolerance for stringbeds that spray the ball. For me, it's only a few hours. Some people play with it for 4-6 hours before cutting it out, some even 10 hours (which is usually pushing it). And some people just keep playing with it, not realizing that the reason they are missing shots is that their stringbed is beyond its useful life. The lack of control of a "dead" poly stringbed is most obvious when you try to volley against an opponent who hits with heavy spin - the dwell time is too long and rebound angle is very difficult to control.
The one neat feature of poly compared to other strings is that it is slippery, which makes the strings slide over each other, making it possible to hit with more spin when you use poly crosses. But any advantage from this increased spin is fleeting -- your control will begin to suffer after a few hours. Don't be drawn in.
The reason poly is so popular with pro's is that they only use their racquets for an hour before they grab a freshly strung one. And sadly, the reason it's so popular with amateurs is because they want to use the same string the pros use.
No pros use kevlar, but's only because they don't use strings long enough to reach the sweet plateau of broken-in kevlar at the right tension. But in my opinion, kevlar is the most underrated, unloved, underappreciated type of string. It gives unmatched control, and nearly as much spin as fresh poly.
After playing 10 years with Problend, then another 5 years with Crossfire, I have played the past few years with kevlar/poly (which gives a little bit more spin than kevlar/syn gut or kevlar/gut). But the frustrating downside is that the poly loses tension so fast. I've currently found an upgrade to poly cross -- Ashaway Zyex Monogut. At the right tension, it will give you almost as much the extra spin of a poly cross, but with much, much, much better tension maintenance (after a boardy break-in period).
I'm also beginning to experiment with thoroughly pre-stretched poly as cross (which in theory has much improved tension stability). But my sincere opinion (as a connoisseur of precision-control equipment) is there is no reason for a kevlar guy to mess with poly. In my opinion it's an inferior product. I'm pretty much done using poly in any situation unless it's thoroughly pre-stretched.
