There's a boatload of great resources on this topic. Here are a few recommendations to get you started:
Install the EE Debug Toolbar
Adam's suggestion of enabling template debugging is a good one. This add-on is a great way to uncover potential issues. Be on the lookout for high query counts and process bottlenecks. There isn't a hard and fast rule for what's excessive, as it depends largely on the functionality and content structure of your particular site / page.
Make Sure You're Using Appropriate Caching Where Possible
If you're relatively new to EE, this one is huge.
Review the Answers for Related EE StackExchange Questions
Add-ons and front-end performance
What are the recommended ways to get to the bottom of slow load times in EE?
Do a Google Search for EE Performance
Check Out Stash
There are a 101 uses for Stash, most of which will help keep your template code more dry and efficient. For starters, I'd recommend checking out Stash Variables, which are fairly simple to wrap your head around (some of the Stash concepts can get complex quickly) and can be super useful in a variety of situations.
Give this Podcast on EE Performance a Listen
There are some great tips in here.
Make Sure the Non-EE Parts of Your Site are Optimized
If you haven't done so already, it might be worth running your site through Google's PageSpeed tool. Generally speaking, it's a good idea to make sure you're combining and minifying your JS and CSS assets, optimizing your images, and reducing the number of http requests wherever possible.