How long is a session token valid?

[bnl]

Hi!
I got some bots that are long running.
They log in (currently via api6) and keep that token.
One bot asks for new markets every minute, and dispatches its reply, if any to the other bots that are interested.
Now, I get this in the logs:

grep INVALID_SESSION_INFORMATION *

markets_fetcher.log:2013-09-07 08:22:54.100:{"id":15, "jsonrpc":"2.0", "error":{"code":-32099, "data":{"exceptionname":"APINGException", "APINGException":{"requestUUID":"prdang006-08021457-0019c1b6f3", "errorCode":"INVALID_SESSION_INFORMATION", "errorDetails":"The session token passed is invalid"}}, "message":"ANGX-0003"}}
markets_fetcher.log:2013-09-09 07:45:50.876:{"id":15, "jsonrpc":"2.0", "error":{"code":-32099, "data":{"exceptionname":"APINGException", "APINGException":{"requestUUID":"prdang002-08021442-001e83ec72", "errorCode":"INVALID_SESSION_INFORMATION", "errorDetails":"The session token passed is invalid"}}, "message":"ANGX-0003"}}
markets_fetcher.log:2013-09-11 08:23:54.168:{"id":15, "jsonrpc":"2.0", "error":{"code":-32099, "data":{"exceptionname":"APINGException", "APINGException":{"requestUUID":"prdang008-09091146-0001e65109", "errorCode":"INVALID_SESSION_INFORMATION", "errorDetails":"The session token passed is invalid"}}, "message":"ANGX-0003"}}

which tells me that a token lasts for about 48 hours.
Why? The bot sends and and get replies using the same token
every minute, so it is not a keepAlive issue.

Is there a recommended time to logout/login ?

/Björn