I'm designing a low-grade chronic treatment with this cytokine and I'm wondering how regularly to change the culture medium based on the half-life of the protein in an in vitro environment.
So I am not entirely certain which form your TNFa is in (soluble or full-length), but I recommend the following paper. While it is investigating albumin tagged TNF, they make comparisons to non-tagged forms which may contain the answer you are looking for.
Article Superior serum half life of albumin tagged TNF ligands
Müller, N., Schneider, B., Pfizenmaier, K., & Wajant, H. (2010). Superior serum half life of albumin tagged TNF ligands. Biochemical and biophysical research communications, 396(4), 793-799.
Alternatively, you can use the ProtParam Tool from ExPASy. It predicts the half-life from a primary amino acid sequence.
https://web.expasy.org/protparam/
Gasteiger, E., Hoogland, C., Gattiker, A., Wilkins, M. R., Appel, R. D., & Bairoch, A. (2005). Protein identification and analysis tools on the ExPASy server. The proteomics protocols handbook, 571-607.