How should I store time series in mongodb
I suggest a single time series entry per document. There are some problems with storing multiple entries per document:


a single document is limited to a certain size (currently 16 MB); this limits how many entries can be stored in a single document
as more entries are added to a document, the entire document (and time series) will needlessly be deleted and reallocated to a larger piece of memory
queries on sub-documents are limited compared to queries on regular documents
documents with very flat structures (like one sub-document for each second) are not performant
the built-in map-reduce does not work as well on sub-documents


Also note a timestamp is built-in to the default MongoDB ObjectId. You can use this if the time series precision is less than one second. 

Here is an example BSON document from an event logging library that uses MongoDB:

Example format of generated bson document:
{
    'thread': -1216977216,
    'level': 'ERROR',
    'timestamp': Timestamp(1290895671, 63),
    'message': 'test message',
    'fileName': '/var/projects/python/log4mongo-python/tests/test_mongo_handler.py',
    'lineNumber': 38,
    'method': 'test_emit_exception',
    'loggerName':  'testLogger',
    'exception': {
        'stackTrace': 'Traceback (most recent call last):
                       File "/var/projects/python/log4mongo-python/tests/test_mongo_handler.py", line 36, in test_emit_exception
                       raise Exception(\'exc1\')
                       Exception: exc1',
        'message': 'exc1',
        'code': 0
    }
}


Since an event log is similar to a time series, it may be worth studying the rest of the code. There are versions in Java, C#, PHP, and Python.

Here is another similar open source project: Zarkov



[update] In response to @RockScience's comment, I've adding some more references:


Very verbose case study of how MongoDB was used to store time series. Interesting parts:


Slide 25: how much data? 20 4-byte data points @ 1 minute intervals for 10 years (not apparent if these are actual statistics or just projected estimates)
Slide 66: shows sample MongoDB document used to store time series data

Best practices for storing log data, straight from the creators of MongoDB