The iOS Messages app writes timestamps to selected incoming and outgoing SMS, MMS and iMessage notes. Selected because Apple has designed Messages to only write the timestamp after a certain amount of time has passed. This can result in long threads with no record of when the messages were sent.
Now thanks to the SMS Timestamps tweak on Cydia, you can choose to write a timestamp to every message if desired. The tweak also has options to write a timestamp in the Messages app in intervals of 1, 5 or 10 minutes so you know exactly what's going on.
To find SMS Timestamps open Cydia and navigate to Sections -> Tweaks or use the Search function to find the tweak. The modification comes from developer Gabrio Barbieri via the BigBoss repository at no charge. This free mod requires a jailbroken iOS device.
Once installed, SMS Timestamps has its own area under Settings -> SMS TImestamps. The tweak can be enabled or disabled and the timestamp frequency is adjustable to "Every message" or the time intervals mentioned above.