I've been playing guitar for almost 7 years now and I'd say i'm decent at it. But the thing is : here in Rome local bands are very rarely looking for guitarists but there are always spots for bassists and drummers, so I've decided to try the bassist path and just yesterday I bought myself a Peavey Millennium 4 bxp (it isn't here yet, it's being shipped atm). Now considering that I've some guitar experience I was wondering : Is it realistic expecting to become a ''decent'' bassist in a fairly short time ?
Good move picking up bass guitar. The more instruments you can play the more opportunities you will have!

Guitar definitely helps in the sense bass will feel familiar. You just have to get used to the size difference, bass is BIG when you are used to guitar lol. It's sort of like going from driving a car to driving a big truck. You need a little more strength for bass as well. Guitar helps with the callouses on your fret hand but your string hand will have to start from the beginning. And because bass strings are so much thicker, expect little helmets to form on your fingers lol.
Other than that, you need to have a strong sense of rhythm and perfect timing.

To answer your question, yes I think you will be able to become a decent bassist quite quickly because of your background in guitar.
I play guitar and when I was writing songs I just used my brothers bass and played no problem. I used a pick, so it was pretty much the same as guitar but easier. Should be no problem.
Gibson SG Special - Tony Iommi pickups
Martin 000CXEBLACK acoustic/electric

Lefties Unite
Wait for the bass to get here, play and find out, noobie. :P
Main Basses:
Warwick RB Corvette 5
Douglas Sculptor 825 NA Fretless 5

Main Rig:
Sansamp>GK Backline 600>2 GK BLX 210 Cabinets