Qeng Ho units of time
JavaScript (ES6) 255
f=s=>(s=s.replace(/(\d+) (..)/g,(_,n,u)=>t+={se:1,mi:60,ho:3600,da:86400,we:604800,mo:2592e3,ye:31536e3,de:31536e4,ce:31536e5}[u]*n,t=0),[a,b]=t>=1e9?[t/1e9,' Gsec']:t>=1e6?[t/1e6,' Msec']:t>999?[t/1e3,' Ksec']:[t,' second'],a.toFixed(3)+b+(a-1?'s':''))
// test
console.log=x=>O.innerHTML+=x+'\n'
;[
['1 hour','3.600 Ksecs']
,['1 day','86.400 Ksecs']
,['2 weeks','1.210 Msecs']
,['1 year','31.536 Msecs']
,['32 years','1.009 Gsecs']
,['1 second','1.000 second']
,['1 century 6 decades','5.046 Gsecs']
,['255 centuries','804.168 Gsecs']
,['2 weeks 6 days 1 hour 19 minutes 4 seconds','1.733 Msecs']
,['1 week 3 days 3 hours 7 minutes','875.220 Ksecs']
,['1 week 4 days 13 hours 46 minutes 40 seconds', '1.000 Msec']
,['2 months 2 hours', '5.191 Msecs']
,['16 minutes 39 seconds', '999 seconds']
].forEach(t=>console.log(t[0]+' '+f(t[0])+' (Check:'+t[1]+')'))<pre id=O></pre>APL (Dyalog APL), 157 156 154 151 154 141 142 bytes
{∊(3⍕N)' '((B/S⊃' KMG')'sec','ond'/⍨~B←T≥1E3),'s'/⍨1≠N←T÷1E3*S←⌊1E3⍟T←+/×/↑⍎¨'\d+ .a?i?'⎕S'&'⊢⍵⊣c←10×d←10×⊃y m w←365 30 7×da←24×h←×⍨mi←60×s←1}
Thanks to ngn for shaving off 13 bytes.
Must have ⎕IO←0, which is default in many APLs.
Try it online!
Python, 366 363 bytes
d={};l=1;q=str.replace;i=q(raw_input(),"ie","y")
for u,t in zip('second minute hour day week month year decade century'.split(),(1,60,60,24,7,30./7,73./6,10,10)):l=t*l;d[u]=d[u+"s"]=l
while" "in i:
i=q(q(i," ","*",1)," ","+",1)
q=eval(i,d);f={};l=1
for u in('second','Ksec','Msec','Gsec'):
l*=1e3
if q<l:q=q*1e3/l;print"%.3f %s%s"%(q,u,("s","")[q<1.001]);break