I recently received the I-130 approval and I was wondering whether I can get started with preparing the I-864 form and assembling the support documents like employment letter, tax forms etc...
Is there any requirement for the employment letter such as expiration or other. Your help is much appreciated!

There is no expiration date for the employment letter. In fact, if you request it now from your HR department, you should get it in time for when you have to submit it along with the I-864.