It will go down gradually, however there are many things that factor into your hub score. If you are active on forums for instance that will help keep your profile score up (only as 1 of many examples.)
Here is how HP staff have it worded in their FAQ for individual hubs:
HubScore takes into account many factors and will change over time as data about its performance continues to stream in. It will also fluctuate quite a bit, seemingly at random, and that is nothing to worry about. The most important HubScore components that you should be concerned with include:
Amount of traffic - including percentage from reputable sources other than HubPages
The reputation of the Hubber - your Hubber Score and contribution to the community
The response of readers to your hub - including comments, thumbs up, etc.
The uniqueness of content - copying content already available on the web will be penalized
We reserve the right to change the factors used to assess a Hub’s HubScore as we continue to determine those that measure a great Hub. The bottom line is if you write original, useful content, your HubScores will reflect the quality and work you've put into them.
I hope this helps :)