The SMS Timestamps Cydia tweak is your best bet if you are looking for a tweak to provide standalone timestamp functionality without other customization options for the Messages.app. With the SMS Timestamps Cydia tweak you can modify the Messages.app’s default behaviour of setting timestamps on the beginning of an active conversation and then ceasing to do it until it deems necessary again. Meaning you can either set timestamps to always be enabled, or to be displayed after a certain amount of time (specified by you).
If you are interested in the SMS Timestamps Cydia tweak you can find it through the BigBoss repo for FREE. If you do not have a tweak that already enables timestamp functionality I would extremely recommend this tweak! Please let us know what you think of the tweak in the comments.