Try
Total Commander.
Go down a little to
Download the Pocket PC version (Freeware!):, download the
CAB version for
ARM/XSCALE/PXA and transfer it into your device.
Install by tapping on it in
File Explorer.
After installing TC;
1 ) Start
Total Commander.

2 ) Double tap on
\\Plugins.

3 ) Double tap on
registry.

4 ) Double tap on
HKLM.

5 ) Double tap on
System.

6 ) Double tap on
Inbox.

7 ) Double tap on
Settings.

8 ) Double tap on
PropertyPath.
If it's not there, double tab on
+Add value+, to add that string.

9 ) Put in the value:
\name of your storage card\Messaging and tap
OK.

10 ) Exit TC, wait 30 seconds and restart your device.
Alternatively are there other periods of time I can set for how many emails to save. The options only go up to one month, after that all emails. If I could choose 3 or 6 months it would be ok.
I can't do that, but you can try this work around.
1 ) In your Outlook, create a new folder. From your Inbox, choose those messages that you don't want to sync to your PPC and move them into this new folder.
2 ) On your device, choose to sync all emails.
3 ) Connect your device to PC and sync them.
4 ) After that, you can delete the new folder from your PPC.