My questions are, how long does the trial version of 7 last?
30 Days
Would i be better off just keeping on using the full version 5.51?
Features to skinning Windows 7 are not available in WB 5.51
Do I have to repurchase each time there is an update to wb?
From Stardock's FAQ's
Q: Do I have to pay for updates?
A: Generally not. The Object Desktop Network includes 12 months of updates and new components. Minor version updates to other products (like WindowBlinds) are made available to registered users at no charge. For instance, if you purchased WindowBlinds 1.0 you would have had access to over 6 updates at no charge. Only major version changes (like moving from WindowBlinds 1.x to WindowBlinds 2.0) will have an upgrade charge.