It is best to submit your trademark as early as possible. Currently, the average processing time is more than 2 months. With the first new gTLDs arriving this fall, it is important to have your trademark validated in order to join the Sunrise, or in order to be notified during Trademark Claims periods.
Also, currently (summer 2013) the Trademark Clearinghouse is not forcing the 20-days deadline for submission of corrections. That means that there is more time to correct any deficiencies. After the summer the Trademark Clearinghouse expects more trademarks to validate, and they will start forcing the 20-days deadline.
The validity of your trademark submission will start immediately, but the Trademark Clearinghouse will extend based on the first new gTLD Sunrise: if you order a 3-year submission of your trademark and this trademark is validated in August 2013, the initial validity period is August 2013 to August 2016. But if the first Sunrise starts in October 2013, the validity period will be extended until October 2016. This period, until the date of the first new gTLD Sunrise, is called Early Bird period.