I'm pretty sure this is a repost but with some search i couldn't find anything similar.
Copy & Paste the following script at your chat window or macro it.
Code:/script p="player";x=UnitXP(p);m=UnitXPMax(p);r=GetXPExhaustion();if -1==(r or -1)then t="No rest."else t="Rest: "..(math.floor(20*r/m+0.5)).." bars ("if r+x<m then t=t..r else t=t.."level +"..(r+x-m)end t=t.."XP)"end;DEFAULT_CHAT_FRAME:AddMessage(t)