Electronic – Circuit Charging Sealed Lead-Acid Battery (NP1270M)

See my stack exchange answer to "Lead Acid Battery Charger Design Factors" which relates, and follow the link there to the Battery University site which will tell you far more than you knew there was to know about lead acid (and other) batteries.

From the above answer note the quotes from the above website. Especially in this context

• The correct setting of the charge voltage is critical and
  ranges from 2.30 to 2.45V per cell.
  Setting the voltage threshold is a compromise, and battery experts refer to this as “dancing on the head of a needle.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, an over-saturated condition causes grid corrosion on the positive plate and induces gassing.

  To make “dancing on the head of a needle” more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly lower voltage thresholds and a cold ambient prefers a higher level.

2.30 x 6 = 13.8V

2.45 x 6 = 14.7V

13.8V is the nominal voltage that many automobile systems operate at.
14.7V is higher than you'd usually meet but around 14.4V is common for "topping" mode charging which is used to equalise charge in series connected cells. See the Battery University site for MUCH more information.

If you charge to only 12.6V as several people have recommended your battery will not ever be at full capacity and will have a shortened life.

See also Safe operating area for different types of battery chemistry?

And also Can I charge a 12v sealed lead acid with an old wall-wart (not made for charging)?

And also Charging lead-acid batteries?

Battery University is a good starting point. A vast amount here Battery University

Particularly:

Charging Lead Acid

Based on the following, it would SOUND wise to charge fully for "a while" under trickle and then to open circuit the charger until Vbattery fell to approaching 2.1V/cell. A controller to implement this could be extremely simple.

Dangerous in isolation, but, they say:

• Most stationary batteries are kept on float charge. To reduce stress, the so-called hysteresis charge disconnects the float current when the battery is full. As the terminal voltage drops due to self-discharge, an occasional topping charge replenishes the lost energy. In essence, the battery is only “borrowed” from time to time for brief moments. This mode works well for installations that do not draw a load when on standby.

  Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell. With AGM, these requirements can be somewhat relaxed.

  Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation.

  Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage.