The plugin "Map Rate" has a similar feature, where the player is invited to rate a map after having played it for a little while. I think you should check out how FLOOR_MASTER achieves this in his Map Rate plugin:
http://forums.alliedmods.net/showthread.php?p=607727
A simple method would be to save the server time when the player joins then hook to some game event with a function that checks to see is a defined time interval has passed since that played joined. This would not require any sort of "wait" feature. Plus, it would allow you to bring up the welcome menu in coordination with some context where the player would not be disrupted by it popping up. For example, hook to Event_PlayerDeath and call the welcome menu up a couple seconds after the player's first death.
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