and how long will it take? also, how much is usually the amount for a new cable? thanks

 

I've had pretty good results lubing the speedo cables when they start to get noisy or jumpy. Most auto supply sotres also have a speedo cabnle lube "kit". It's basically an aerosol lube with an adapter that allows you to pressure lube the cable from the tranny end (the easier end to get off for most people & cars). It's easy to DIY tho, either by applying vacuum to one end of the cable & sucking the lube in from the other end or forcing it in under prssure by aerosol can from one end. The key is to get the lube to go clear up or down the cable. Don't prssure lube from the bottom without making sure your not "oiling" your guage cluster tho!
New cable run $25-50, but they're a PITA to install without buggering 'em up (i.e. no bends/kinks, firewall or body leaks) I'd try the lube trick forst at least
Lubinh takes an hour or so & most of that time is making adapters for vacuums or aerosol cans etc. R2 of a cable looks like about twice that although I'm sure it can be done quicker with experience. (lubing has always done the trick for me, never replaced one myself)