Just how big can a site be on a shared hosting plan?

[Cedik]

I know that I'm not the first and neither the last to put this problem but still, I'd like to know in advance so I won't be having troubles later.
I have a baby plan and recently I've been starting to use more and more resources.
This is caused mostly by one of my sites, who was using quite some bandwidth. Still, I'm not talking about very big figures here.
So far, I've been using about 50 Gb of bandwidth monthly (with all of my sites) and this month I may reach 60 Gb. However, I already started to optimize my site and reduce the bandwidth it uses so the next month I should drop under 50 Gb again. Anyhow, I had no problems with the host and no warnings but I like to be cautious.
The "biggest" site that I have is a portal+forum which currently has about 650 members. I get around 500 visitors a day, of which about 50% are direct traffic. The rest comes from referrers and search engines. According to Awstats, I get about 1000 visits per day and I might reach 20.000 unique visitors monthly soon enough. It consumes about 30, maybe 40 Gb per month.
It's not that big yet but it will get bigger. My question is how much bigger can it get before I should start looking for other hosting options, like VPS or even a dedicated server?
Also, moving from Baby to Swamp might help me in the future? The Swamp plan allows more resources to be consumed by the user? And by this I mean resources like the CPU and not the Bandwidth and disk space.

Will a Hostgator Shared account still hold me when I'll have 1000 unique visitors a day and I'll use up to... 60-80 Gb monthly? (these are figures for just one site, not for my whole account)

[Goddess Dix]

If you're pushing the limits now, why not upgrade now? At least to a bigger plan until the site(s) can financially support a dedicated server.

[Cedik]

Well, that's the problem. I don't know if it's pushing the limits or even if it reaches them. I only know that it starts to use more bandwidth than before. And looking at other hosting companies plans, I can see that 60Gb per month with only 10$ is more than a great deal. True, Hostgator is bigger than other hosting companies but still...
My budget is quite low so I'd rather stick with this plan for as long as it is possible. However... just how long is it possible? That's what concerns me.
As I understood, sites that goes over 2-3k visitors a day should move from a shared account. Though, this varies from site to site. Still, I'm pretty certain that there must be at least a theoretical limit for a shared hosting site, speaking in terms of common sense, that is.
How should a site know when it's time to move forward?
When the bandwidth it uses goes over a certain value? When the numbers of daily visitors surpasses another value? When it often has over a certain number of simultaneous on-line visitors? And so on...
That's pretty much what I would like to know.

[Goddess Dix]

well, it's not straight visitors; the limits you're most likely to hit are cpu limits, and that's going to vary a LOT from site to site. mostly database calls and script execution increase load on the server. so lots of visitors to static pages will have less impact that fewer visitors to a db-driven site.

perhaps somebody else can give you a ballpark idea, but honestly, a lot depends on your specific site.

[Cedik]

Well, I run a forum+portal combination...
I'm bothering you with one final question though: If I'll buy a reseller account and use it as a regular account? Will that help me?