When will I be able to download Catalina on my iMac 2011?

No.

Never...

macOS Catalina - Technical Specifications

General Requirements

▪ OS X 10.9 or later

▪ 4GB of memory

▪ 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

▪ Some features require an Apple ID; terms apply.

▪ Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:

▪ MacBook (Early 2015 or newer)

▪ MacBook Air (Mid 2012 or newer)

▪ MacBook Pro (Mid 2012 or newer)

▪ Mac mini (Late 2012 or newer)

▪ iMac (Late 2012 or newer)

▪ iMac Pro (2017)

▪ Mac Pro (Late 2013 or newer)

If you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

To tell you the truth, I wouldn't be too anxious to update to Catalina. I have it on my iMac and MacBook and am thinking about going back to Mohave. I have never had so many issues with trying to move music to the new app, ejecting external drives, and problems with Photos. I am not sure it's worth it!!

The reason why not is because Catalina requires a GPU that supports a graphics functionality called Metal. Metal became available in iMacs in 2012. Hence why a 2011 will not work,before you ask no it is not possible to upgrade a 2011 and no there are no workarounds.

The issues you are having are very likely due to what you have installed on your system. If you would like to create a separate thread please do and include an EtreCheck report of your system and we can look for obvious issues.

I do have separate threads for the issues. The latest one about Photos only has one reply: I am having the same issue. Thanks!