What is the longest time a transaction has been in limbo for in Bitcoin?
This question cannot be answered reliably.
The only part of Bitcoin that is publicly known is the blockchain. The transactions that are confirmed are on the blockchain, that is shared online.
The transaction is "stuck in limbo" - unconfirmed - from the moment it is created to the moment it is in blockchain. However, you cannot tell reliably when it was created.
Each node tracks unconfirmed transactions, and it can track where it saw the transaction first, but it doesn't really mean when it was created.
Blockchain.info shows this as "Received time". You could probably count answer to your question from that, but it wouldn't be reliable.
I can add a personal answer: I run my own node and sometimes look on mempool, what is the oldest transaction there; today, I saw 6 days old transactions in mempool. So, the longest will be surely more than 6 days. But that is just an incomplete, personal observation. :)
Bitcoin is about taking proposed changes to a data set (transactions submitted to the blockchain), and having a decentralized network come to consensus on which changes are accepted in which order. You are asking about the proposed changes, which by definition means that they are not yet agreed upon. As such, nobody can tell you which changes are "in limbo", because we haven't agreed on which ones count. If I run 5 nodes in my basement, and they lost connectivity to the rest of the network, do transactions in their mempool count? What happens when they reconnect?
In Bitcoin, nothing really counts until consensus is reached.