Try to download the file at night. The traffic at NCBI is bad and you only get 20-30 KB/s. The entire human proteome is about 250MB (450000 sequences x 500Bytes/sequence) and it will take a long time to download at this rate. I am not sure NCBI has a time limit.
I tried in the morning and it downloaded about 90000 sequences in 3 hours and it was still going.