Hypothetically, if you only have one node mining in the network, how much time does it take to solve for the nonce assuming for example the current difficulty?
The number of nodes is irrelevant. What matters is how much hashpower you have; i.e. what kind of hardware is attached to that node, and how rapidly it can perform SHA256D hashes. If your node is an ordinary computer with no specialized ASIC hardware, then at today's difficulty levels, for all practical purposes, you will never mine a block (on average, it will take several million years to do so).
See https://en.bitcoin.it/wiki/Difficulty#How_soon_might_I_expect_to_generate_a_block.3F to learn about how to estimate the time to mine one block given your hashpower.