I'm assuming the Charisma-modifier-times-per-day limit on using the rogue talent underhanded does not apply to the +4 circumstance bonus to Sleight of Hand skill checks made to conceal a weapon, which means for making devastating sneak attacks
The Talent Is Useless Outside the Surprise Round
Even if the rogue has not yet been perceived by his foes, if the surprise round is over, the rogue won't benefit from the rogue talent underhanded's increased sneak attack damage. Even if his foes remain flat-footed after the surprise round, the rogue won't benefit from the rogue talent underhanded's increased sneak attack damage during the first regular round of combat.
Here's more about the surprise round:
If some but not all of the combatants are aware of their opponents, a surprise round happens before regular rounds begin. In initiative order (highest to lowest), combatants who started the battle aware of their opponents each take a standard or move action during the surprise round. You can also take free actions during the surprise round. If no one or everyone is surprised, no surprise round occurs.
To use the rogue talent underhanded for more than just a bonus to Sleight of Hand skill checks to conceal a weapon, the rogue must during the surprise round make a sneak attack with a weapon the foe was unaware of.
The rogue talent underhanded has spawned at least two different threads, one on the Paizo forums and one on the Giant in the Playground forums. Both are essential reads for understanding this talent's limits. The act of drawing a concealed weapon during the surprise round--almost a necessity for using this talent and also a difficult task--is discussed in those threads at length, also.