First, turn on the Answering system, either with the base Answer On/Off button or the handset (Ans. Setup, Ans. On/Off). Next make sure that the memory isn't full. If the message LED reads FL, delete old messages with the handset. You'll need to press the button with the envelope/Mute on it to access the message.
Now, press Menu/Select, scroll through the menu to Ans. Setup. Select this and search for Record Time. Select this and choose the recording time of 1 or 4 minutes. Press Menu/Select again to save the setting.
This is in the manual on page 45. You can get the manual from Uniden at https://www.uniden.com/product-information/owners-manuals
. Search for the DECT1580 (the last 5 indicates the number of handsets). The direct link to the manual is https://www.uniden.com/File%20Library/FooterNav/Product%20Information/Owners%20Manuals/DECT1580om.pdf
I hope this helps.