Once the tracking snippet is installed, it will begin collecting data going forward. If you are seeing anything that looks off, it's probably because you're including a date range from before the snippet was on your site.
There are three issues you may encounter for a short period until enough time has passed and you have a more comprehensive data set.
1. My trends aren't working - I'm not seeing the green arrows pointing up or the red arrows pointing down.
Let's say your date range is set to 30 days. The trends work by comparing that data to the previous 30 days. If we don't have any data from the previous 30 days, you won't get trend information because there's nothing to compare it to.
2. I'm not getting any data for Avg Time on Page and/or Bounce Rate.
This data only starts getting collected once you've installed the snippet. If you've selected a date range prior to the day that the snippet was installed, we won't have any data.
3. I'm seeing unusually long/short numbers for Avg Time on Page or Bounce Rates.
If your snippet was only installed recently, there is a small sample size of visitors going to your website. Those numbers will normalize over time.