For general statements about being tired at work or being sick of work will be covered in the 'Talking to coworkers' section. These types of statements shouldn't be said to everyone.

"I'm busy with work."
"Can I do this later?"
"Does this have to be done now?"
"When do you need this by?"
"When is the deadline?"

"I'm not going to have time for any other assignment until Thursday."

"I'll try to free up my schedule."
"I'm pretty sure I can meet the deadline."
"If I don't get any help on this, I won't be able to complete it on time."
"I'll do whatever I can to make sure this is done on time."

"You can count on me."
"Have I ever let you down?"